Convert the full parser into constexpr now that Abseil requires C++14, and use

this parser for the static checker.
This fixes some outstanding bugs where the static checker differed from the
dynamic one.
Also, fix `%v` to be accepted with POSIX syntax.

Tested:
  Presubmit
  TGP OCL:487237262:BASE:490275393:1669141454896:92dd62e3
PiperOrigin-RevId: 491650577
Change-Id: Id138c108187428b3aea46f8887495f1da12c91b2

6 files changed, 412 insertions, 624 deletions

diff --git a/absl/strings/internal/str_format/checker.h b/absl/strings/internal/str_format/checker.h
index aeb9d48d..eab6ab9d 100644
--- a/absl/strings/internal/str_format/checker.h
+++ b/absl/strings/internal/str_format/checker.h
@@ -15,8 +15,11 @@
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 
+#include <algorithm>
+
 #include "absl/base/attributes.h"
 #include "absl/strings/internal/str_format/arg.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
 #include "absl/strings/internal/str_format/extension.h"
 
 // Compile time check support for entry points.
@@ -36,333 +39,56 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-constexpr bool AllOf() { return true; }
-
-template <typename... T>
-constexpr bool AllOf(bool b, T... t) {
-  return b && AllOf(t...);
-}
-
 #ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-constexpr bool ContainsChar(const char* chars, char c) {
-  return *chars == c || (*chars && ContainsChar(chars + 1, c));
-}
-
-// A constexpr compatible list of Convs.
-struct ConvList {
-  const FormatConversionCharSet* array;
-  int count;
-
-  // We do the bound check here to avoid having to do it on the callers.
-  // Returning an empty FormatConversionCharSet has the same effect as
-  // short circuiting because it will never match any conversion.
-  constexpr FormatConversionCharSet operator[](int i) const {
-    return i < count ? array[i] : FormatConversionCharSet{};
-  }
-
-  constexpr ConvList without_front() const {
-    return count != 0 ? ConvList{array + 1, count - 1} : *this;
-  }
-};
-
-template <size_t count>
-struct ConvListT {
-  // Make sure the array has size > 0.
-  FormatConversionCharSet list[count ? count : 1];
-};
-
-constexpr char GetChar(string_view str, size_t index) {
-  return index < str.size() ? str[index] : char{};
-}
-
-constexpr string_view ConsumeFront(string_view str, size_t len = 1) {
-  return len <= str.size() ? string_view(str.data() + len, str.size() - len)
-                           : string_view();
-}
-
-constexpr string_view ConsumeAnyOf(string_view format, const char* chars) {
-  while (ContainsChar(chars, GetChar(format, 0))) {
-    format = ConsumeFront(format);
-  }
-  return format;
-}
-
-constexpr bool IsDigit(char c) { return c >= '0' && c <= '9'; }
-
-// Helper class for the ParseDigits function.
-// It encapsulates the two return values we need there.
-struct Integer {
-  string_view format;
-  int value;
-
-  // If the next character is a '$', consume it.
-  // Otherwise, make `this` an invalid positional argument.
-  constexpr Integer ConsumePositionalDollar() const {
-    if (GetChar(format, 0) == '$') {
-      return Integer{ConsumeFront(format), value};
-    } else {
-      return Integer{format, 0};
-    }
-  }
-};
-
-constexpr Integer ParseDigits(string_view format) {
-  int value = 0;
-  while (IsDigit(GetChar(format, 0))) {
-    value = 10 * value + GetChar(format, 0) - '0';
-    format = ConsumeFront(format);
-  }
-
-  return Integer{format, value};
-}
-
-// Parse digits for a positional argument.
-// The parsing also consumes the '$'.
-constexpr Integer ParsePositional(string_view format) {
-  return ParseDigits(format).ConsumePositionalDollar();
-}
-
-// Parses a single conversion specifier.
-// See ConvParser::Run() for post conditions.
-class ConvParser {
-  constexpr ConvParser SetFormat(string_view format) const {
-    return ConvParser(format, args_, error_, arg_position_, is_positional_);
-  }
-
-  constexpr ConvParser SetArgs(ConvList args) const {
-    return ConvParser(format_, args, error_, arg_position_, is_positional_);
-  }
-
-  constexpr ConvParser SetError(bool error) const {
-    return ConvParser(format_, args_, error_ || error, arg_position_,
-                      is_positional_);
-  }
-
-  constexpr ConvParser SetArgPosition(int arg_position) const {
-    return ConvParser(format_, args_, error_, arg_position, is_positional_);
-  }
-
-  // Consumes the next arg and verifies that it matches `conv`.
-  // `error_` is set if there is no next arg or if it doesn't match `conv`.
-  constexpr ConvParser ConsumeNextArg(char conv) const {
-    return SetArgs(args_.without_front()).SetError(!Contains(args_[0], conv));
-  }
-
-  // Verify that positional argument `i.value` matches `conv`.
-  // `error_` is set if `i.value` is not a valid argument or if it doesn't
-  // match.
-  constexpr ConvParser VerifyPositional(Integer i, char conv) const {
-    return SetFormat(i.format).SetError(!Contains(args_[i.value - 1], conv));
-  }
-
-  // Parse the position of the arg and store it in `arg_position_`.
-  constexpr ConvParser ParseArgPosition(Integer arg) const {
-    return SetFormat(arg.format).SetArgPosition(arg.value);
-  }
-
-  // Consume the flags.
-  constexpr ConvParser ParseFlags() const {
-    return SetFormat(ConsumeAnyOf(format_, "-+ #0"));
-  }
-
-  // Consume the width.
-  // If it is '*', we verify that it matches `args_`. `error_` is set if it
-  // doesn't match.
-  constexpr ConvParser ParseWidth() const {
-    char first_char = GetChar(format_, 0);
-
-    if (IsDigit(first_char)) {
-      return SetFormat(ParseDigits(format_).format);
-    } else if (first_char == '*') {
-      if (is_positional_) {
-        return VerifyPositional(ParsePositional(ConsumeFront(format_)), '*');
-      } else {
-        return SetFormat(ConsumeFront(format_)).ConsumeNextArg('*');
-      }
-    } else {
-      return *this;
+template <FormatConversionCharSet... C>
+constexpr bool ValidFormatImpl(string_view format) {
+  int next_arg = 0;
+  const char* p = format.data();
+  const char* const end = p + format.size();
+  constexpr FormatConversionCharSet
+      kAllowedConvs[(std::max)(sizeof...(C), size_t{1})] = {C...};
+  bool used[(std::max)(sizeof...(C), size_t{1})]{};
+  constexpr int kNumArgs = sizeof...(C);
+  while (p != end) {
+    while (p != end && *p != '%') ++p;
+    if (p == end) {
+      break;
     }
-  }
-
-  // Consume the precision.
-  // If it is '*', we verify that it matches `args_`. `error_` is set if it
-  // doesn't match.
-  constexpr ConvParser ParsePrecision() const {
-    if (GetChar(format_, 0) != '.') {
-      return *this;
-    } else if (GetChar(format_, 1) == '*') {
-      if (is_positional_) {
-        return VerifyPositional(ParsePositional(ConsumeFront(format_, 2)), '*');
-      } else {
-        return SetFormat(ConsumeFront(format_, 2)).ConsumeNextArg('*');
-      }
-    } else {
-      return SetFormat(ParseDigits(ConsumeFront(format_)).format);
+    if (p + 1 >= end) return false;
+    if (p[1] == '%') {
+      // %%
+      p += 2;
+      continue;
     }
-  }
-
-  // Consume the length characters.
-  constexpr ConvParser ParseLength() const {
-    return SetFormat(ConsumeAnyOf(format_, "lLhjztq"));
-  }
-
-  // Consume the conversion character and verify that it matches `args_`.
-  // `error_` is set if it doesn't match.
-  constexpr ConvParser ParseConversion() const {
-    char first_char = GetChar(format_, 0);
 
-    if (first_char == 'v' && *(format_.data() - 1) != '%') {
-      return SetError(true);
+    UnboundConversion conv(absl::kConstInit);
+    p = ConsumeUnboundConversion(p + 1, end, &conv, &next_arg);
+    if (p == nullptr) return false;
+    if (conv.arg_position <= 0 || conv.arg_position > kNumArgs) {
+      return false;
     }
-
-    if (is_positional_) {
-      return VerifyPositional({ConsumeFront(format_), arg_position_},
-                              first_char);
-    } else {
-      return ConsumeNextArg(first_char).SetFormat(ConsumeFront(format_));
+    if (!Contains(kAllowedConvs[conv.arg_position - 1], conv.conv)) {
+      return false;
     }
-  }
-
-  constexpr ConvParser(string_view format, ConvList args, bool error,
-                       int arg_position, bool is_positional)
-      : format_(format),
-        args_(args),
-        error_(error),
-        arg_position_(arg_position),
-        is_positional_(is_positional) {}
-
- public:
-  constexpr ConvParser(string_view format, ConvList args, bool is_positional)
-      : format_(format),
-        args_(args),
-        error_(false),
-        arg_position_(0),
-        is_positional_(is_positional) {}
-
-  // Consume the whole conversion specifier.
-  // `format()` will be set to the character after the conversion character.
-  // `error()` will be set if any of the arguments do not match.
-  constexpr ConvParser Run() const {
-    ConvParser parser = *this;
-
-    if (is_positional_) {
-      parser = ParseArgPosition(ParsePositional(format_));
-    }
-
-    return parser.ParseFlags()
-        .ParseWidth()
-        .ParsePrecision()
-        .ParseLength()
-        .ParseConversion();
-  }
-
-  constexpr string_view format() const { return format_; }
-  constexpr ConvList args() const { return args_; }
-  constexpr bool error() const { return error_; }
-  constexpr bool is_positional() const { return is_positional_; }
-
- private:
-  string_view format_;
-  // Current list of arguments. If we are not in positional mode we will consume
-  // from the front.
-  ConvList args_;
-  bool error_;
-  // Holds the argument position of the conversion character, if we are in
-  // positional mode. Otherwise, it is unspecified.
-  int arg_position_;
-  // Whether we are in positional mode.
-  // It changes the behavior of '*' and where to find the converted argument.
-  bool is_positional_;
-};
-
-// Parses a whole format expression.
-// See FormatParser::Run().
-class FormatParser {
-  static constexpr bool FoundPercent(string_view format) {
-    return format.empty() ||
-           (GetChar(format, 0) == '%' && GetChar(format, 1) != '%');
-  }
-
-  // We use an inner function to increase the recursion limit.
-  // The inner function consumes up to `limit` characters on every run.
-  // This increases the limit from 512 to ~512*limit.
-  static constexpr string_view ConsumeNonPercentInner(string_view format) {
-    int limit = 20;
-    while (!FoundPercent(format) && limit != 0) {
-      size_t len = 0;
-
-      if (GetChar(format, 0) == '%' && GetChar(format, 1) == '%') {
-        len = 2;
-      } else {
-        len = 1;
+    used[conv.arg_position - 1] = true;
+    for (auto extra : {conv.width, conv.precision}) {
+      if (extra.is_from_arg()) {
+        int pos = extra.get_from_arg();
+        if (pos <= 0 || pos > kNumArgs) return false;
+        used[pos - 1] = true;
+        if (!Contains(kAllowedConvs[pos - 1], '*')) {
+          return false;
+        }
       }
-
-      format = ConsumeFront(format, len);
-      --limit;
     }
-
-    return format;
   }
-
-  // Consume characters until the next conversion spec %.
-  // It skips %%.
-  static constexpr string_view ConsumeNonPercent(string_view format) {
-    while (!FoundPercent(format)) {
-      format = ConsumeNonPercentInner(format);
+  if (sizeof...(C) != 0) {
+    for (bool b : used) {
+      if (!b) return false;
     }
-
-    return format;
-  }
-
-  static constexpr bool IsPositional(string_view format) {
-    while (IsDigit(GetChar(format, 0))) {
-      format = ConsumeFront(format);
-    }
-
-    return GetChar(format, 0) == '$';
   }
-
-  constexpr bool RunImpl(bool is_positional) const {
-    // In non-positional mode we require all arguments to be consumed.
-    // In positional mode just reaching the end of the format without errors is
-    // enough.
-    return (format_.empty() && (is_positional || args_.count == 0)) ||
-           (!format_.empty() &&
-            ValidateArg(
-                ConvParser(ConsumeFront(format_), args_, is_positional).Run()));
-  }
-
-  constexpr bool ValidateArg(ConvParser conv) const {
-    return !conv.error() && FormatParser(conv.format(), conv.args())
-                                .RunImpl(conv.is_positional());
-  }
-
- public:
-  constexpr FormatParser(string_view format, ConvList args)
-      : format_(ConsumeNonPercent(format)), args_(args) {}
-
-  // Runs the parser for `format` and `args`.
-  // It verifies that the format is valid and that all conversion specifiers
-  // match the arguments passed.
-  // In non-positional mode it also verfies that all arguments are consumed.
-  constexpr bool Run() const {
-    return RunImpl(!format_.empty() && IsPositional(ConsumeFront(format_)));
-  }
-
- private:
-  string_view format_;
-  // Current list of arguments.
-  // If we are not in positional mode we will consume from the front and will
-  // have to be empty in the end.
-  ConvList args_;
-};
-
-template <FormatConversionCharSet... C>
-constexpr bool ValidFormatImpl(string_view format) {
-  return FormatParser(format,
-                      {ConvListT<sizeof...(C)>{{C...}}.list, sizeof...(C)})
-      .Run();
+  return true;
 }
 
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
diff --git a/absl/strings/internal/str_format/checker_test.cc b/absl/strings/internal/str_format/checker_test.cc
index 680517f7..a86bed38 100644
--- a/absl/strings/internal/str_format/checker_test.cc
+++ b/absl/strings/internal/str_format/checker_test.cc
@@ -93,6 +93,7 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<void (*)(), volatile int*>("%p %p"),  //
       ValidFormat<string_view, const char*, double, void*>(
           "string_view=%s const char*=%s double=%f void*=%p)"),
+      ValidFormat<int>("%v"),  //
 
       ValidFormat<int>("%% %1$d"),               //
       ValidFormat<int>("%1$ld"),                 //
@@ -109,7 +110,9 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int, double>("%2$.*1$f"),      //
       ValidFormat<void*, string_view, const char*, double>(
           "string_view=%2$s const char*=%3$s double=%4$f void*=%1$p "
-          "repeat=%3$s)")};
+          "repeat=%3$s)"),
+      ValidFormat<std::string>("%1$v"),
+  };
 
   for (Case c : trues) {
     EXPECT_TRUE(c.result) << c.format;
@@ -130,6 +133,8 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int>("%*d"),               //
       ValidFormat<std::string>("%p"),        //
       ValidFormat<int (*)(int)>("%d"),       //
+      ValidFormat<int>("%1v"),               //
+      ValidFormat<int>("%.1v"),              //
 
       ValidFormat<>("%3$d"),                     //
       ValidFormat<>("%1$r"),                     //
@@ -138,13 +143,14 @@ TEST(StrFormatChecker, ValidFormat) {
       ValidFormat<int>("%1$*2$1d"),              //
       ValidFormat<int>("%1$1-d"),                //
       ValidFormat<std::string, int>("%2$*1$s"),  //
-      ValidFormat<std::string>("%1$p"),
+      ValidFormat<std::string>("%1$p"),          //
+      ValidFormat<int>("%1$*2$v"),               //
 
       ValidFormat<int, int>("%d %2$d"),  //
   };
 
   for (Case c : falses) {
-    EXPECT_FALSE(c.result) << c.format;
+    EXPECT_FALSE(c.result) << "format<" << c.format << ">";
   }
 }
 
diff --git a/absl/strings/internal/str_format/constexpr_parser.h b/absl/strings/internal/str_format/constexpr_parser.h
new file mode 100644
index 00000000..3dc1776b
--- /dev/null
+++ b/absl/strings/internal/str_format/constexpr_parser.h
@@ -0,0 +1,351 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
+#define ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
+
+#include <cassert>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/const_init.h"
+#include "absl/strings/internal/str_format/extension.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace str_format_internal {
+
+enum class LengthMod : std::uint8_t { h, hh, l, ll, L, j, z, t, q, none };
+
+// The analyzed properties of a single specified conversion.
+struct UnboundConversion {
+  // This is a user defined default constructor on purpose to skip the
+  // initialization of parts of the object that are not necessary.
+  UnboundConversion() {}  // NOLINT
+
+  // This constructor is provided for the static checker. We don't want to do
+  // the unnecessary initialization in the normal case.
+  explicit constexpr UnboundConversion(absl::ConstInitType)
+      : arg_position{}, width{}, precision{} {}
+
+  class InputValue {
+   public:
+    constexpr void set_value(int value) {
+      assert(value >= 0);
+      value_ = value;
+    }
+    constexpr int value() const { return value_; }
+
+    // Marks the value as "from arg". aka the '*' format.
+    // Requires `value >= 1`.
+    // When set, is_from_arg() return true and get_from_arg() returns the
+    // original value.
+    // `value()`'s return value is unspecified in this state.
+    constexpr void set_from_arg(int value) {
+      assert(value > 0);
+      value_ = -value - 1;
+    }
+    constexpr bool is_from_arg() const { return value_ < -1; }
+    constexpr int get_from_arg() const {
+      assert(is_from_arg());
+      return -value_ - 1;
+    }
+
+   private:
+    int value_ = -1;
+  };
+
+  // No need to initialize. It will always be set in the parser.
+  int arg_position;
+
+  InputValue width;
+  InputValue precision;
+
+  Flags flags = Flags::kBasic;
+  LengthMod length_mod = LengthMod::none;
+  FormatConversionChar conv = FormatConversionCharInternal::kNone;
+};
+
+// Helper tag class for the table below.
+// It allows fast `char -> ConversionChar/LengthMod/Flags` checking and
+// conversions.
+class ConvTag {
+ public:
+  constexpr ConvTag(FormatConversionChar conversion_char)  // NOLINT
+      : tag_(static_cast<uint8_t>(conversion_char)) {}
+  constexpr ConvTag(LengthMod length_mod)  // NOLINT
+      : tag_(0x80 | static_cast<uint8_t>(length_mod)) {}
+  constexpr ConvTag(Flags flags)  // NOLINT
+      : tag_(0xc0 | static_cast<uint8_t>(flags)) {}
+  constexpr ConvTag() : tag_(0xFF) {}
+
+  constexpr bool is_conv() const { return (tag_ & 0x80) == 0; }
+  constexpr bool is_length() const { return (tag_ & 0xC0) == 0x80; }
+  constexpr bool is_flags() const { return (tag_ & 0xE0) == 0xC0; }
+
+  constexpr FormatConversionChar as_conv() const {
+    assert(is_conv());
+    assert(!is_length());
+    assert(!is_flags());
+    return static_cast<FormatConversionChar>(tag_);
+  }
+  constexpr LengthMod as_length() const {
+    assert(!is_conv());
+    assert(is_length());
+    assert(!is_flags());
+    return static_cast<LengthMod>(tag_ & 0x3F);
+  }
+  constexpr Flags as_flags() const {
+    assert(!is_conv());
+    assert(!is_length());
+    assert(is_flags());
+    return static_cast<Flags>(tag_ & 0x1F);
+  }
+
+ private:
+  uint8_t tag_;
+};
+
+struct ConvTagHolder {
+  using CC = FormatConversionCharInternal;
+  using LM = LengthMod;
+
+  // Abbreviations to fit in the table below.
+  static constexpr auto kFSign = Flags::kSignCol;
+  static constexpr auto kFAlt = Flags::kAlt;
+  static constexpr auto kFPos = Flags::kShowPos;
+  static constexpr auto kFLeft = Flags::kLeft;
+  static constexpr auto kFZero = Flags::kZero;
+
+  static constexpr ConvTag value[256] = {
+      {},     {},    {},    {},    {},    {},     {},    {},     // 00-07
+      {},     {},    {},    {},    {},    {},     {},    {},     // 08-0f
+      {},     {},    {},    {},    {},    {},     {},    {},     // 10-17
+      {},     {},    {},    {},    {},    {},     {},    {},     // 18-1f
+      kFSign, {},    {},    kFAlt, {},    {},     {},    {},     //  !"#$%&'
+      {},     {},    {},    kFPos, {},    kFLeft, {},    {},     // ()*+,-./
+      kFZero, {},    {},    {},    {},    {},     {},    {},     // 01234567
+      {},     {},    {},    {},    {},    {},     {},    {},     // 89:;<=>?
+      {},     CC::A, {},    {},    {},    CC::E,  CC::F, CC::G,  // @ABCDEFG
+      {},     {},    {},    {},    LM::L, {},     {},    {},     // HIJKLMNO
+      {},     {},    {},    {},    {},    {},     {},    {},     // PQRSTUVW
+      CC::X,  {},    {},    {},    {},    {},     {},    {},     // XYZ[\]^_
+      {},     CC::a, {},    CC::c, CC::d, CC::e,  CC::f, CC::g,  // `abcdefg
+      LM::h,  CC::i, LM::j, {},    LM::l, {},     CC::n, CC::o,  // hijklmno
+      CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  CC::v, {},     // pqrstuvw
+      CC::x,  {},    LM::z, {},    {},    {},     {},    {},     // xyz{|}!
+      {},     {},    {},    {},    {},    {},     {},    {},     // 80-87
+      {},     {},    {},    {},    {},    {},     {},    {},     // 88-8f
+      {},     {},    {},    {},    {},    {},     {},    {},     // 90-97
+      {},     {},    {},    {},    {},    {},     {},    {},     // 98-9f
+      {},     {},    {},    {},    {},    {},     {},    {},     // a0-a7
+      {},     {},    {},    {},    {},    {},     {},    {},     // a8-af
+      {},     {},    {},    {},    {},    {},     {},    {},     // b0-b7
+      {},     {},    {},    {},    {},    {},     {},    {},     // b8-bf
+      {},     {},    {},    {},    {},    {},     {},    {},     // c0-c7
+      {},     {},    {},    {},    {},    {},     {},    {},     // c8-cf
+      {},     {},    {},    {},    {},    {},     {},    {},     // d0-d7
+      {},     {},    {},    {},    {},    {},     {},    {},     // d8-df
+      {},     {},    {},    {},    {},    {},     {},    {},     // e0-e7
+      {},     {},    {},    {},    {},    {},     {},    {},     // e8-ef
+      {},     {},    {},    {},    {},    {},     {},    {},     // f0-f7
+      {},     {},    {},    {},    {},    {},     {},    {},     // f8-ff
+  };
+};
+
+// Keep a single table for all the conversion chars and length modifiers.
+constexpr ConvTag GetTagForChar(char c) {
+  return ConvTagHolder::value[static_cast<unsigned char>(c)];
+}
+
+constexpr bool CheckFastPathSetting(const UnboundConversion& conv) {
+  bool width_precision_needed =
+      conv.width.value() >= 0 || conv.precision.value() >= 0;
+  if (width_precision_needed && conv.flags == Flags::kBasic) {
+#if defined(__clang__)
+    // Some compilers complain about this in constexpr even when not executed,
+    // so only enable the error dump in clang.
+    fprintf(stderr,
+            "basic=%d left=%d show_pos=%d sign_col=%d alt=%d zero=%d "
+            "width=%d precision=%d\n",
+            conv.flags == Flags::kBasic ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kLeft) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kShowPos) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kSignCol) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kAlt) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kZero) ? 1 : 0, conv.width.value(),
+            conv.precision.value());
+#endif  // defined(__clang__)
+    return false;
+  }
+  return true;
+}
+
+constexpr int ParseDigits(char& c, const char*& pos, const char* const end) {
+  int digits = c - '0';
+  // We do not want to overflow `digits` so we consume at most digits10
+  // digits. If there are more digits the parsing will fail later on when the
+  // digit doesn't match the expected characters.
+  int num_digits = std::numeric_limits<int>::digits10;
+  for (;;) {
+    if (ABSL_PREDICT_FALSE(pos == end)) break;
+    c = *pos++;
+    if ('0' > c || c > '9') break;
+    --num_digits;
+    if (ABSL_PREDICT_FALSE(!num_digits)) break;
+    digits = 10 * digits + c - '0';
+  }
+  return digits;
+}
+
+template <bool is_positional>
+constexpr const char* ConsumeConversion(const char* pos, const char* const end,
+                                        UnboundConversion* conv,
+                                        int* next_arg) {
+  const char* const original_pos = pos;
+  char c = 0;
+  // Read the next char into `c` and update `pos`. Returns false if there are
+  // no more chars to read.
+#define ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR()          \
+  do {                                                  \
+    if (ABSL_PREDICT_FALSE(pos == end)) return nullptr; \
+    c = *pos++;                                         \
+  } while (0)
+
+  if (is_positional) {
+    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+    conv->arg_position = ParseDigits(c, pos, end);
+    assert(conv->arg_position > 0);
+    if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
+  }
+
+  ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+
+  // We should start with the basic flag on.
+  assert(conv->flags == Flags::kBasic);
+
+  // Any non alpha character makes this conversion not basic.
+  // This includes flags (-+ #0), width (1-9, *) or precision (.).
+  // All conversion characters and length modifiers are alpha characters.
+  if (c < 'A') {
+    while (c <= '0') {
+      auto tag = GetTagForChar(c);
+      if (tag.is_flags()) {
+        conv->flags = conv->flags | tag.as_flags();
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+      } else {
+        break;
+      }
+    }
+
+    if (c <= '9') {
+      if (c >= '0') {
+        int maybe_width = ParseDigits(c, pos, end);
+        if (!is_positional && c == '$') {
+          if (ABSL_PREDICT_FALSE(*next_arg != 0)) return nullptr;
+          // Positional conversion.
+          *next_arg = -1;
+          return ConsumeConversion<true>(original_pos, end, conv, next_arg);
+        }
+        conv->flags = conv->flags | Flags::kNonBasic;
+        conv->width.set_value(maybe_width);
+      } else if (c == '*') {
+        conv->flags = conv->flags | Flags::kNonBasic;
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        if (is_positional) {
+          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+          conv->width.set_from_arg(ParseDigits(c, pos, end));
+          if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
+          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        } else {
+          conv->width.set_from_arg(++*next_arg);
+        }
+      }
+    }
+
+    if (c == '.') {
+      conv->flags = conv->flags | Flags::kNonBasic;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+      if ('0' <= c && c <= '9') {
+        conv->precision.set_value(ParseDigits(c, pos, end));
+      } else if (c == '*') {
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        if (is_positional) {
+          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
+          conv->precision.set_from_arg(ParseDigits(c, pos, end));
+          if (c != '$') return nullptr;
+          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+        } else {
+          conv->precision.set_from_arg(++*next_arg);
+        }
+      } else {
+        conv->precision.set_value(0);
+      }
+    }
+  }
+
+  auto tag = GetTagForChar(c);
+
+  if (ABSL_PREDICT_FALSE(c == 'v' && conv->flags != Flags::kBasic)) {
+    return nullptr;
+  }
+
+  if (ABSL_PREDICT_FALSE(!tag.is_conv())) {
+    if (ABSL_PREDICT_FALSE(!tag.is_length())) return nullptr;
+
+    // It is a length modifier.
+    using str_format_internal::LengthMod;
+    LengthMod length_mod = tag.as_length();
+    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    if (c == 'h' && length_mod == LengthMod::h) {
+      conv->length_mod = LengthMod::hh;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    } else if (c == 'l' && length_mod == LengthMod::l) {
+      conv->length_mod = LengthMod::ll;
+      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+    } else {
+      conv->length_mod = length_mod;
+    }
+    tag = GetTagForChar(c);
+
+    if (ABSL_PREDICT_FALSE(c == 'v')) return nullptr;
+    if (ABSL_PREDICT_FALSE(!tag.is_conv())) return nullptr;
+  }
+
+  assert(CheckFastPathSetting(*conv));
+  (void)(&CheckFastPathSetting);
+
+  conv->conv = tag.as_conv();
+  if (!is_positional) conv->arg_position = ++*next_arg;
+  return pos;
+}
+
+// Consume conversion spec prefix (not including '%') of [p, end) if valid.
+// Examples of valid specs would be e.g.: "s", "d", "-12.6f".
+// If valid, it returns the first character following the conversion spec,
+// and the spec part is broken down and returned in 'conv'.
+// If invalid, returns nullptr.
+constexpr const char* ConsumeUnboundConversion(const char* p, const char* end,
+                                               UnboundConversion* conv,
+                                               int* next_arg) {
+  if (*next_arg < 0) return ConsumeConversion<true>(p, end, conv, next_arg);
+  return ConsumeConversion<false>(p, end, conv, next_arg);
+}
+
+}  // namespace str_format_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_CONSTEXPR_PARSER_H_
diff --git a/absl/strings/internal/str_format/parser.cc b/absl/strings/internal/str_format/parser.cc
index 13731ee2..5aaab698 100644
--- a/absl/strings/internal/str_format/parser.cc
+++ b/absl/strings/internal/str_format/parser.cc
@@ -31,211 +31,14 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-using CC = FormatConversionCharInternal;
-using LM = LengthMod;
+// Define the array for non-constexpr uses.
+constexpr ConvTag ConvTagHolder::value[256];
 
-// Abbreviations to fit in the table below.
-constexpr auto f_sign = Flags::kSignCol;
-constexpr auto f_alt = Flags::kAlt;
-constexpr auto f_pos = Flags::kShowPos;
-constexpr auto f_left = Flags::kLeft;
-constexpr auto f_zero = Flags::kZero;
-
-ABSL_CONST_INIT const ConvTag kTags[256] = {
-    {},     {},    {},    {},    {},    {},     {},    {},     // 00-07
-    {},     {},    {},    {},    {},    {},     {},    {},     // 08-0f
-    {},     {},    {},    {},    {},    {},     {},    {},     // 10-17
-    {},     {},    {},    {},    {},    {},     {},    {},     // 18-1f
-    f_sign, {},    {},    f_alt, {},    {},     {},    {},     //  !"#$%&'
-    {},     {},    {},    f_pos, {},    f_left, {},    {},     // ()*+,-./
-    f_zero, {},    {},    {},    {},    {},     {},    {},     // 01234567
-    {},     {},    {},    {},    {},    {},     {},    {},     // 89:;<=>?
-    {},     CC::A, {},    {},    {},    CC::E,  CC::F, CC::G,  // @ABCDEFG
-    {},     {},    {},    {},    LM::L, {},     {},    {},     // HIJKLMNO
-    {},     {},    {},    {},    {},    {},     {},    {},     // PQRSTUVW
-    CC::X,  {},    {},    {},    {},    {},     {},    {},     // XYZ[\]^_
-    {},     CC::a, {},    CC::c, CC::d, CC::e,  CC::f, CC::g,  // `abcdefg
-    LM::h,  CC::i, LM::j, {},    LM::l, {},     CC::n, CC::o,  // hijklmno
-    CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  CC::v, {},     // pqrstuvw
-    CC::x,  {},    LM::z, {},    {},    {},     {},    {},     // xyz{|}!
-    {},     {},    {},    {},    {},    {},     {},    {},     // 80-87
-    {},     {},    {},    {},    {},    {},     {},    {},     // 88-8f
-    {},     {},    {},    {},    {},    {},     {},    {},     // 90-97
-    {},     {},    {},    {},    {},    {},     {},    {},     // 98-9f
-    {},     {},    {},    {},    {},    {},     {},    {},     // a0-a7
-    {},     {},    {},    {},    {},    {},     {},    {},     // a8-af
-    {},     {},    {},    {},    {},    {},     {},    {},     // b0-b7
-    {},     {},    {},    {},    {},    {},     {},    {},     // b8-bf
-    {},     {},    {},    {},    {},    {},     {},    {},     // c0-c7
-    {},     {},    {},    {},    {},    {},     {},    {},     // c8-cf
-    {},     {},    {},    {},    {},    {},     {},    {},     // d0-d7
-    {},     {},    {},    {},    {},    {},     {},    {},     // d8-df
-    {},     {},    {},    {},    {},    {},     {},    {},     // e0-e7
-    {},     {},    {},    {},    {},    {},     {},    {},     // e8-ef
-    {},     {},    {},    {},    {},    {},     {},    {},     // f0-f7
-    {},     {},    {},    {},    {},    {},     {},    {},     // f8-ff
-};
-
-namespace {
-
-bool CheckFastPathSetting(const UnboundConversion& conv) {
-  bool width_precision_needed =
-      conv.width.value() >= 0 || conv.precision.value() >= 0;
-  if (width_precision_needed && conv.flags == Flags::kBasic) {
-    fprintf(stderr,
-            "basic=%d left=%d show_pos=%d sign_col=%d alt=%d zero=%d "
-            "width=%d precision=%d\n",
-            conv.flags == Flags::kBasic ? 1 : 0,
-            FlagsContains(conv.flags, Flags::kLeft) ? 1 : 0,
-            FlagsContains(conv.flags, Flags::kShowPos) ? 1 : 0,
-            FlagsContains(conv.flags, Flags::kSignCol) ? 1 : 0,
-            FlagsContains(conv.flags, Flags::kAlt) ? 1 : 0,
-            FlagsContains(conv.flags, Flags::kZero) ? 1 : 0, conv.width.value(),
-            conv.precision.value());
-    return false;
-  }
-  return true;
-}
-
-template <bool is_positional>
-const char *ConsumeConversion(const char *pos, const char *const end,
-                              UnboundConversion *conv, int *next_arg) {
-  const char* const original_pos = pos;
-  char c;
-  // Read the next char into `c` and update `pos`. Returns false if there are
-  // no more chars to read.
-#define ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR()          \
-  do {                                                  \
-    if (ABSL_PREDICT_FALSE(pos == end)) return nullptr; \
-    c = *pos++;                                         \
-  } while (0)
-
-  const auto parse_digits = [&] {
-    int digits = c - '0';
-    // We do not want to overflow `digits` so we consume at most digits10
-    // digits. If there are more digits the parsing will fail later on when the
-    // digit doesn't match the expected characters.
-    int num_digits = std::numeric_limits<int>::digits10;
-    for (;;) {
-      if (ABSL_PREDICT_FALSE(pos == end)) break;
-      c = *pos++;
-      if (!std::isdigit(c)) break;
-      --num_digits;
-      if (ABSL_PREDICT_FALSE(!num_digits)) break;
-      digits = 10 * digits + c - '0';
-    }
-    return digits;
-  };
-
-  if (is_positional) {
-    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-    conv->arg_position = parse_digits();
-    assert(conv->arg_position > 0);
-    if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
-  }
-
-  ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-
-  // We should start with the basic flag on.
-  assert(conv->flags == Flags::kBasic);
-
-  // Any non alpha character makes this conversion not basic.
-  // This includes flags (-+ #0), width (1-9, *) or precision (.).
-  // All conversion characters and length modifiers are alpha characters.
-  if (c < 'A') {
-    while (c <= '0') {
-      auto tag = GetTagForChar(c);
-      if (tag.is_flags()) {
-        conv->flags = conv->flags | tag.as_flags();
-        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-      } else {
-        break;
-      }
-    }
-
-    if (c <= '9') {
-      if (c >= '0') {
-        int maybe_width = parse_digits();
-        if (!is_positional && c == '$') {
-          if (ABSL_PREDICT_FALSE(*next_arg != 0)) return nullptr;
-          // Positional conversion.
-          *next_arg = -1;
-          return ConsumeConversion<true>(original_pos, end, conv, next_arg);
-        }
-        conv->flags = conv->flags | Flags::kNonBasic;
-        conv->width.set_value(maybe_width);
-      } else if (c == '*') {
-        conv->flags = conv->flags | Flags::kNonBasic;
-        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        if (is_positional) {
-          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-          conv->width.set_from_arg(parse_digits());
-          if (ABSL_PREDICT_FALSE(c != '$')) return nullptr;
-          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        } else {
-          conv->width.set_from_arg(++*next_arg);
-        }
-      }
-    }
-
-    if (c == '.') {
-      conv->flags = conv->flags | Flags::kNonBasic;
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-      if (std::isdigit(c)) {
-        conv->precision.set_value(parse_digits());
-      } else if (c == '*') {
-        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        if (is_positional) {
-          if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
-          conv->precision.set_from_arg(parse_digits());
-          if (c != '$') return nullptr;
-          ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-        } else {
-          conv->precision.set_from_arg(++*next_arg);
-        }
-      } else {
-        conv->precision.set_value(0);
-      }
-    }
-  }
-
-  auto tag = GetTagForChar(c);
-
-  if (ABSL_PREDICT_FALSE(c == 'v' && (pos - original_pos) != 1)) return nullptr;
-
-  if (ABSL_PREDICT_FALSE(!tag.is_conv())) {
-    if (ABSL_PREDICT_FALSE(!tag.is_length())) return nullptr;
-
-    // It is a length modifier.
-    using str_format_internal::LengthMod;
-    LengthMod length_mod = tag.as_length();
-    ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    if (c == 'h' && length_mod == LengthMod::h) {
-      conv->length_mod = LengthMod::hh;
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    } else if (c == 'l' && length_mod == LengthMod::l) {
-      conv->length_mod = LengthMod::ll;
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    } else {
-      conv->length_mod = length_mod;
-    }
-    tag = GetTagForChar(c);
-
-    if (ABSL_PREDICT_FALSE(c == 'v')) return nullptr;
-    if (ABSL_PREDICT_FALSE(!tag.is_conv())) return nullptr;
-  }
-
-  assert(CheckFastPathSetting(*conv));
-  (void)(&CheckFastPathSetting);
-
-  conv->conv = tag.as_conv();
-  if (!is_positional) conv->arg_position = ++*next_arg;
-  return pos;
+ABSL_ATTRIBUTE_NOINLINE const char* ConsumeUnboundConversionNoInline(
+    const char* p, const char* end, UnboundConversion* conv, int* next_arg) {
+  return ConsumeUnboundConversion(p, end, conv, next_arg);
 }
 
-}  // namespace
-
 std::string LengthModToString(LengthMod v) {
   switch (v) {
     case LengthMod::h:
@@ -262,12 +65,6 @@ std::string LengthModToString(LengthMod v) {
   return "";
 }
 
-const char *ConsumeUnboundConversion(const char *p, const char *end,
-                                     UnboundConversion *conv, int *next_arg) {
-  if (*next_arg < 0) return ConsumeConversion<true>(p, end, conv, next_arg);
-  return ConsumeConversion<false>(p, end, conv, next_arg);
-}
-
 struct ParsedFormatBase::ParsedFormatConsumer {
   explicit ParsedFormatConsumer(ParsedFormatBase *parsedformat)
       : parsed(parsedformat), data_pos(parsedformat->data_.get()) {}
diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
index a81bac83..35b6d49c 100644
--- a/absl/strings/internal/str_format/parser.h
+++ b/absl/strings/internal/str_format/parser.h
@@ -29,111 +29,18 @@
 #include <vector>
 
 #include "absl/strings/internal/str_format/checker.h"
+#include "absl/strings/internal/str_format/constexpr_parser.h"
 #include "absl/strings/internal/str_format/extension.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-enum class LengthMod : std::uint8_t { h, hh, l, ll, L, j, z, t, q, none };
-
 std::string LengthModToString(LengthMod v);
 
-// The analyzed properties of a single specified conversion.
-struct UnboundConversion {
-  UnboundConversion() {}
-
-  class InputValue {
-   public:
-    void set_value(int value) {
-      assert(value >= 0);
-      value_ = value;
-    }
-    int value() const { return value_; }
-
-    // Marks the value as "from arg". aka the '*' format.
-    // Requires `value >= 1`.
-    // When set, is_from_arg() return true and get_from_arg() returns the
-    // original value.
-    // `value()`'s return value is unspecfied in this state.
-    void set_from_arg(int value) {
-      assert(value > 0);
-      value_ = -value - 1;
-    }
-    bool is_from_arg() const { return value_ < -1; }
-    int get_from_arg() const {
-      assert(is_from_arg());
-      return -value_ - 1;
-    }
-
-   private:
-    int value_ = -1;
-  };
-
-  // No need to initialize. It will always be set in the parser.
-  int arg_position;
-
-  InputValue width;
-  InputValue precision;
-
-  Flags flags = Flags::kBasic;
-  LengthMod length_mod = LengthMod::none;
-  FormatConversionChar conv = FormatConversionCharInternal::kNone;
-};
-
-// Consume conversion spec prefix (not including '%') of [p, end) if valid.
-// Examples of valid specs would be e.g.: "s", "d", "-12.6f".
-// If valid, it returns the first character following the conversion spec,
-// and the spec part is broken down and returned in 'conv'.
-// If invalid, returns nullptr.
-const char* ConsumeUnboundConversion(const char* p, const char* end,
-                                     UnboundConversion* conv, int* next_arg);
-
-// Helper tag class for the table below.
-// It allows fast `char -> ConversionChar/LengthMod/Flags` checking and
-// conversions.
-class ConvTag {
- public:
-  constexpr ConvTag(FormatConversionChar conversion_char)  // NOLINT
-      : tag_(static_cast<uint8_t>(conversion_char)) {}
-  constexpr ConvTag(LengthMod length_mod)  // NOLINT
-      : tag_(0x80 | static_cast<uint8_t>(length_mod)) {}
-  constexpr ConvTag(Flags flags)  // NOLINT
-      : tag_(0xc0 | static_cast<uint8_t>(flags)) {}
-  constexpr ConvTag() : tag_(0xFF) {}
-
-  bool is_conv() const { return (tag_ & 0x80) == 0; }
-  bool is_length() const { return (tag_ & 0xC0) == 0x80; }
-  bool is_flags() const { return (tag_ & 0xE0) == 0xC0; }
-
-  FormatConversionChar as_conv() const {
-    assert(is_conv());
-    assert(!is_length());
-    assert(!is_flags());
-    return static_cast<FormatConversionChar>(tag_);
-  }
-  LengthMod as_length() const {
-    assert(!is_conv());
-    assert(is_length());
-    assert(!is_flags());
-    return static_cast<LengthMod>(tag_ & 0x3F);
-  }
-  Flags as_flags() const {
-    assert(!is_conv());
-    assert(!is_length());
-    assert(is_flags());
-    return static_cast<Flags>(tag_ & 0x1F);
-  }
-
- private:
-  uint8_t tag_;
-};
-
-extern const ConvTag kTags[256];
-// Keep a single table for all the conversion chars and length modifiers.
-inline ConvTag GetTagForChar(char c) {
-  return kTags[static_cast<unsigned char>(c)];
-}
+const char* ConsumeUnboundConversionNoInline(const char* p, const char* end,
+                                             UnboundConversion* conv,
+                                             int* next_arg);
 
 // Parse the format string provided in 'src' and pass the identified items into
 // 'consumer'.
@@ -187,7 +94,7 @@ bool ParseFormatString(string_view src, Consumer consumer) {
       }
     } else if (percent[1] != '%') {
       UnboundConversion conv;
-      p = ConsumeUnboundConversion(percent + 1, end, &conv, &next_arg);
+      p = ConsumeUnboundConversionNoInline(percent + 1, end, &conv, &next_arg);
       if (ABSL_PREDICT_FALSE(p == nullptr)) return false;
       if (ABSL_PREDICT_FALSE(!consumer.ConvertOne(
               conv, string_view(percent + 1,
diff --git a/absl/strings/internal/str_format/parser_test.cc b/absl/strings/internal/str_format/parser_test.cc
index c3e825fe..021f6a87 100644
--- a/absl/strings/internal/str_format/parser_test.cc
+++ b/absl/strings/internal/str_format/parser_test.cc
@@ -117,6 +117,7 @@ TEST_F(ConsumeUnboundConversionTest, ConsumeSpecification) {
     {__LINE__, "dzz",  "d",    "zz"},  // length mod as suffix
     {__LINE__, "3v",   "",     "3v"},  // 'v' cannot have modifiers
     {__LINE__, "hv",   "",     "hv"},  // 'v' cannot have modifiers
+    {__LINE__, "1$v",   "1$v",     ""},  // 'v' can have use posix syntax
     {__LINE__, "1$*2$d", "1$*2$d", ""  },  // arg indexing and * allowed.
     {__LINE__, "0-14.3hhd", "0-14.3hhd", ""},  // precision, width
     {__LINE__, " 0-+#14.3hhd", " 0-+#14.3hhd", ""},  // flags