From ed71db1e8fc024a3c1b13268653f0a8812c56e34 Mon Sep 17 00:00:00 2001
From: Benjamin Barenblat <bbaren@google.com>
Date: Mon, 17 Jun 2019 10:04:05 -0400
Subject: Use C++11-compatible assertions in constexpr functions

C++11 restricts the bodies of constexpr functions more aggressively than
C++14 or later. Introduce and use a C++11-compatible macro for
assertions in constexpr functions.
---
 src/base/optional.h | 10 ++++------
 src/base/utils.h    | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/src/base/optional.h b/src/base/optional.h
index 5ede4af..79300d5 100644
--- a/src/base/optional.h
+++ b/src/base/optional.h
@@ -16,6 +16,7 @@
 #define ASTC_CODEC_BASE_OPTIONAL_H_
 
 #include "src/base/type_traits.h"
+#include "src/base/utils.h"
 
 #include <cassert>
 #include <initializer_list>
@@ -350,8 +351,7 @@ class Optional
     return get();
   }
   constexpr const T& value() const {
-    assert(this->constructed());
-    return get();
+    return ASTC_CONSTEXPR_ASSERT(this->constructed()), get();
   }
 
   T* ptr() { return this->constructed() ? &get() : nullptr; }
@@ -372,8 +372,7 @@ class Optional
     return get();
   }
   constexpr const T& operator*() const {
-    assert(this->constructed());
-    return get();
+    return ASTC_CONSTEXPR_ASSERT(this->constructed()), get();
   }
 
   T* operator->() {
@@ -381,8 +380,7 @@ class Optional
     return &get();
   }
   constexpr const T* operator->() const {
-    assert(this->constructed());
-    return &get();
+    return ASTC_CONSTEXPR_ASSERT(this->constructed()), &get();
   }
 
   ~Optional() {
diff --git a/src/base/utils.h b/src/base/utils.h
index 0a4fabd..610066b 100644
--- a/src/base/utils.h
+++ b/src/base/utils.h
@@ -32,4 +32,21 @@
 #define UTILS_RELEASE_ASSERT(x) assert(x)
 #endif
 
+// In C++11, `assert` can't be used portably within constexpr functions.
+// ASTC_CONSTEXPR_ASSERT functions as a runtime assert but works in C++11
+// constexpr functions. Example:
+//
+// constexpr double Divide(double a, double b) {
+//   return ASTC_CONSTEXPR_ASSERT(b != 0), a / b;
+// }
+//
+// This macro is based on ABSL_ASSERT.
+#ifdef NDEBUG
+#define ASTC_CONSTEXPR_ASSERT(expr) \
+  (false ? static_cast<void>(expr) : static_cast<void>(0))
+#else
+#define ASTC_CONSTEXPR_ASSERT(expr) \
+  ((expr) ? static_cast<void>(0) : [] { assert(false && #expr); }())
+#endif
+
 #endif  // ASTC_CODEC_BASE_UTILS_H_
-- 
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