Fix tensor casts for large packets and casts to/from std::complex

The original tensor casts were only defined for
`SrcCoeffRatio`:`TgtCoeffRatio` 1:1, 1:2, 2:1, 4:1. Here we add the
missing 1:N and 8:1.

We also add casting `Eigen::half` to/from `std::complex<T>`, which
was missing to make it consistent with `Eigen:bfloat16`, and
generalize the overload to work for any complex type.

Tests were added to `basicstuff`, `packetmath`, and
`cxx11_tensor_casts` to test all cast configurations.

1 files changed, 152 insertions, 0 deletions

diff --git a/test/random_without_cast_overflow.h b/test/random_without_cast_overflow.h
new file mode 100644
index 000000000..000345110
--- /dev/null
+++ b/test/random_without_cast_overflow.h
@@ -0,0 +1,152 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2020 C. Antonio Sanchez <cantonios@google.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// Utilities for generating random numbers without overflows, which might
+// otherwise result in undefined behavior.
+
+namespace Eigen {
+namespace internal {
+
+// Default implementation assuming SrcScalar fits into TgtScalar.
+template <typename SrcScalar, typename TgtScalar, typename EnableIf = void>
+struct random_without_cast_overflow {
+  static SrcScalar value() { return internal::random<SrcScalar>(); }
+};
+
+// Signed to unsigned integer widening cast.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<NumTraits<SrcScalar>::IsInteger && NumTraits<TgtScalar>::IsInteger &&
+                                 !NumTraits<TgtScalar>::IsSigned &&
+                                 (std::numeric_limits<SrcScalar>::digits < std::numeric_limits<TgtScalar>::digits ||
+                                  (std::numeric_limits<SrcScalar>::digits == std::numeric_limits<TgtScalar>::digits &&
+                                   NumTraits<SrcScalar>::IsSigned))>::type> {
+  static SrcScalar value() {
+    SrcScalar a = internal::random<SrcScalar>();
+    return a < SrcScalar(0) ? -(a + 1) : a;
+  }
+};
+
+// Integer to unsigned narrowing cast.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<
+        NumTraits<SrcScalar>::IsInteger && NumTraits<TgtScalar>::IsInteger && !NumTraits<SrcScalar>::IsSigned &&
+        (std::numeric_limits<SrcScalar>::digits > std::numeric_limits<TgtScalar>::digits)>::type> {
+  static SrcScalar value() {
+    TgtScalar b = internal::random<TgtScalar>();
+    return static_cast<SrcScalar>(b < TgtScalar(0) ? -(b + 1) : b);
+  }
+};
+
+// Integer to signed narrowing cast.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<
+        NumTraits<SrcScalar>::IsInteger && NumTraits<TgtScalar>::IsInteger && NumTraits<SrcScalar>::IsSigned &&
+        (std::numeric_limits<SrcScalar>::digits > std::numeric_limits<TgtScalar>::digits)>::type> {
+  static SrcScalar value() { return static_cast<SrcScalar>(internal::random<TgtScalar>()); }
+};
+
+// Unsigned to signed integer narrowing cast.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<NumTraits<SrcScalar>::IsInteger && NumTraits<TgtScalar>::IsInteger &&
+                                 !NumTraits<SrcScalar>::IsSigned && NumTraits<TgtScalar>::IsSigned &&
+                                 (std::numeric_limits<SrcScalar>::digits ==
+                                  std::numeric_limits<TgtScalar>::digits)>::type> {
+  static SrcScalar value() { return internal::random<SrcScalar>() / 2; }
+};
+
+// Floating-point to integer, full precision.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<
+        !NumTraits<SrcScalar>::IsInteger && !NumTraits<SrcScalar>::IsComplex && NumTraits<TgtScalar>::IsInteger &&
+        (std::numeric_limits<TgtScalar>::digits <= std::numeric_limits<SrcScalar>::digits)>::type> {
+  static SrcScalar value() { return static_cast<SrcScalar>(internal::random<TgtScalar>()); }
+};
+
+// Floating-point to integer, narrowing precision.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<
+        !NumTraits<SrcScalar>::IsInteger && !NumTraits<SrcScalar>::IsComplex && NumTraits<TgtScalar>::IsInteger &&
+        (std::numeric_limits<TgtScalar>::digits > std::numeric_limits<SrcScalar>::digits)>::type> {
+  static SrcScalar value() {
+    // NOTE: internal::random<T>() is limited by RAND_MAX, so random<int64_t> is always within that range.
+    // This prevents us from simply shifting bits, which would result in only 0 or -1.
+    // Instead, keep least-significant K bits and sign.
+    static const TgtScalar KeepMask = (static_cast<TgtScalar>(1) << std::numeric_limits<SrcScalar>::digits) - 1;
+    const TgtScalar a = internal::random<TgtScalar>();
+    return static_cast<SrcScalar>(a > TgtScalar(0) ? (a & KeepMask) : -(a & KeepMask));
+  }
+};
+
+// Integer to floating-point, re-use above logic.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<NumTraits<SrcScalar>::IsInteger && !NumTraits<TgtScalar>::IsInteger &&
+                                 !NumTraits<TgtScalar>::IsComplex>::type> {
+  static SrcScalar value() {
+    return static_cast<SrcScalar>(random_without_cast_overflow<TgtScalar, SrcScalar>::value());
+  }
+};
+
+// Floating-point narrowing conversion.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<!NumTraits<SrcScalar>::IsInteger && !NumTraits<SrcScalar>::IsComplex &&
+                                 !NumTraits<TgtScalar>::IsInteger && !NumTraits<TgtScalar>::IsComplex &&
+                                 (std::numeric_limits<SrcScalar>::digits >
+                                  std::numeric_limits<TgtScalar>::digits)>::type> {
+  static SrcScalar value() { return static_cast<SrcScalar>(internal::random<TgtScalar>()); }
+};
+
+// Complex to non-complex.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<NumTraits<SrcScalar>::IsComplex && !NumTraits<TgtScalar>::IsComplex>::type> {
+  typedef typename NumTraits<SrcScalar>::Real SrcReal;
+  static SrcScalar value() { return SrcScalar(random_without_cast_overflow<SrcReal, TgtScalar>::value(), 0); }
+};
+
+// Non-complex to complex.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<!NumTraits<SrcScalar>::IsComplex && NumTraits<TgtScalar>::IsComplex>::type> {
+  typedef typename NumTraits<TgtScalar>::Real TgtReal;
+  static SrcScalar value() { return random_without_cast_overflow<SrcScalar, TgtReal>::value(); }
+};
+
+// Complex to complex.
+template <typename SrcScalar, typename TgtScalar>
+struct random_without_cast_overflow<
+    SrcScalar, TgtScalar,
+    typename internal::enable_if<NumTraits<SrcScalar>::IsComplex && NumTraits<TgtScalar>::IsComplex>::type> {
+  typedef typename NumTraits<SrcScalar>::Real SrcReal;
+  typedef typename NumTraits<TgtScalar>::Real TgtReal;
+  static SrcScalar value() {
+    return SrcScalar(random_without_cast_overflow<SrcReal, TgtReal>::value(),
+                     random_without_cast_overflow<SrcReal, TgtReal>::value());
+  }
+};
+
+}  // namespace internal
+}  // namespace Eigen