From 1b84f21e321e9daa1efcd4422ae92c1782c5582c Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Wed, 22 Jul 2020 18:09:00 -0700
Subject: [PATCH] Revert change that made conversion from bfloat16 to {float,
 double} implicit. Add roundtrip tests for casting between bfloat16 and
 complex types.

---
 Eigen/src/Core/arch/Default/BFloat16.h | 10 +++++----
 test/bfloat16_float.cpp                | 31 +++++++++++++++++---------
 2 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/Eigen/src/Core/arch/Default/BFloat16.h b/Eigen/src/Core/arch/Default/BFloat16.h
index f9c6e76a9..30c998249 100644
--- a/Eigen/src/Core/arch/Default/BFloat16.h
+++ b/Eigen/src/Core/arch/Default/BFloat16.h
@@ -34,8 +34,9 @@ namespace Eigen {
 
 struct bfloat16;
 
-// Since we allow implicit conversion of bfloat16 to float and double, we
-// need to make the cast to complex a bit more explicit
+// explicit conversion operators are no available before C++11 so we first cast
+// bfloat16 to RealScalar rather than to std::complex<RealScalar> directly
+#if !EIGEN_HAS_CXX11
 namespace internal {
 template <typename RealScalar>
 struct cast_impl<bfloat16, std::complex<RealScalar> > {
@@ -45,6 +46,7 @@ struct cast_impl<bfloat16, std::complex<RealScalar> > {
   }
 };
 } // namespace internal
+#endif  // EIGEN_HAS_CXX11
 
 namespace bfloat16_impl {
 
@@ -129,10 +131,10 @@ struct bfloat16 : public bfloat16_impl::bfloat16_base {
   EIGEN_DEVICE_FUNC EIGEN_EXPLICIT_CAST(unsigned long long) const {
     return static_cast<unsigned long long>(bfloat16_to_float(*this));
   }
-  EIGEN_DEVICE_FUNC operator float() const {
+  EIGEN_DEVICE_FUNC EIGEN_EXPLICIT_CAST(float) const {
     return bfloat16_impl::bfloat16_to_float(*this);
   }
-  EIGEN_DEVICE_FUNC operator double() const {
+  EIGEN_DEVICE_FUNC EIGEN_EXPLICIT_CAST(double) const {
     return static_cast<double>(bfloat16_impl::bfloat16_to_float(*this));
   }
   template<typename RealScalar>
diff --git a/test/bfloat16_float.cpp b/test/bfloat16_float.cpp
index 96341929a..11fc31363 100644
--- a/test/bfloat16_float.cpp
+++ b/test/bfloat16_float.cpp
@@ -41,6 +41,19 @@ void test_truncate(float input, float expected_truncation, float expected_roundi
   VERIFY_IS_EQUAL(expected_rounding, static_cast<float>(rounded));
 }
 
+template<typename T>
+ void test_roundtrip() {
+  // Representable T round trip via bfloat16
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(-std::numeric_limits<T>::infinity())), -std::numeric_limits<T>::infinity());
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(std::numeric_limits<T>::infinity())), std::numeric_limits<T>::infinity());
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(-1.0))), T(-1.0));
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(-0.5))), T(-0.5));
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(-0.0))), T(-0.0));
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(1.0))), T(1.0));
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(0.5))), T(0.5));
+  VERIFY_IS_EQUAL(static_cast<T>(static_cast<bfloat16>(T(0.0))), T(0.0));
+}
+
 void test_conversion()
 {
   using Eigen::bfloat16_impl::__bfloat16_raw;
@@ -53,9 +66,9 @@ void test_conversion()
   VERIFY_IS_EQUAL(bfloat16(3.40e38f).value, 0x7f80);  // Becomes infinity.
 
   // Verify round-to-nearest-even behavior.
-  float val1 = bfloat16(__bfloat16_raw(0x3c00));
-  float val2 = bfloat16(__bfloat16_raw(0x3c01));
-  float val3 = bfloat16(__bfloat16_raw(0x3c02));
+  float val1 = static_cast<float>(bfloat16(__bfloat16_raw(0x3c00)));
+  float val2 = static_cast<float>(bfloat16(__bfloat16_raw(0x3c01)));
+  float val3 = static_cast<float>(bfloat16(__bfloat16_raw(0x3c02)));
   VERIFY_IS_EQUAL(bfloat16(0.5f * (val1 + val2)).value, 0x3c00);
   VERIFY_IS_EQUAL(bfloat16(0.5f * (val2 + val3)).value, 0x3c02);
 
@@ -106,14 +119,10 @@ void test_conversion()
   VERIFY_IS_EQUAL(static_cast<float>(bfloat16()), 0.0f);
 
   // Representable floats round trip via bfloat16
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(-std::numeric_limits<float>::infinity())), -std::numeric_limits<float>::infinity());
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(std::numeric_limits<float>::infinity())), std::numeric_limits<float>::infinity());
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(-1.0f)), -1.0f);
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(-0.5f)), -0.5f);
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(-0.0f)), -0.0f);
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(1.0f)), 1.0f);
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(0.5f)), 0.5f);
-  VERIFY_IS_EQUAL(static_cast<float>(static_cast<bfloat16>(0.0f)), 0.0f);
+  test_roundtrip<float>();
+  test_roundtrip<double>();
+  test_roundtrip<std::complex<float> >();
+  test_roundtrip<std::complex<double> >();
 
   // Truncate test
   test_truncate(