Explicit casts of S -> std::complex<T>

When calling `internal::cast<S, std::complex<T>>(x)`, clang often
generates an implicit conversion warning due to an implicit cast
from type `S` to `T`.  This currently affects the following tests:
- `basicstuff`
- `bfloat16_float`
- `cxx11_tensor_casts`

The implicit cast leads to widening/narrowing float conversions.
Widening warnings only seem to be generated by clang (`-Wdouble-promotion`).

To eliminate the warning, we explicitly cast the real-component first
from `S` to `T`.  We also adjust tests to use `internal::cast` instead
of `static_cast` when a complex type may be involved.

1 files changed, 19 insertions, 3 deletions

diff --git a/Eigen/src/Core/MathFunctions.h b/Eigen/src/Core/MathFunctions.h
index 07f4b9493..e9da35995 100644
--- a/Eigen/src/Core/MathFunctions.h
+++ b/Eigen/src/Core/MathFunctions.h
@@ -376,7 +376,7 @@ struct hypot_retval
 * Implementation of cast                                                 *
 ****************************************************************************/
 
-template<typename OldType, typename NewType>
+template<typename OldType, typename NewType, typename EnableIf = void>
 struct cast_impl
 {
   EIGEN_DEVICE_FUNC
@@ -386,6 +386,22 @@ struct cast_impl
   }
 };
 
+// Casting from S -> Complex<T> leads to an implicit conversion from S to T,
+// generating warnings on clang.  Here we explicitly cast the real component.
+template<typename OldType, typename NewType>
+struct cast_impl<OldType, NewType,
+  typename internal::enable_if<
+    !NumTraits<OldType>::IsComplex && NumTraits<NewType>::IsComplex
+  >::type>
+{
+  EIGEN_DEVICE_FUNC
+  static inline NewType run(const OldType& x)
+  {
+    typedef typename NumTraits<NewType>::Real NewReal;
+    return static_cast<NewType>(static_cast<NewReal>(x));
+  }
+};
+
 // here, for once, we're plainly returning NewType: we don't want cast to do weird things.
 
 template<typename OldType, typename NewType>
@@ -486,7 +502,7 @@ struct rint_retval
       #if defined(EIGEN_HIP_DEVICE_COMPILE)
       // HIP does not seem to have a native device side implementation for the math routine "arg"
       using std::arg;
-      #else 		  
+      #else
       EIGEN_USING_STD(arg);
       #endif
       return arg(x);
@@ -967,7 +983,7 @@ template<typename T> T generic_fast_tanh_float(const T& a_x);
 
 namespace numext {
 
-#if (!defined(EIGEN_GPUCC) || defined(EIGEN_CONSTEXPR_ARE_DEVICE_FUNC)) 
+#if (!defined(EIGEN_GPUCC) || defined(EIGEN_CONSTEXPR_ARE_DEVICE_FUNC))
 template<typename T>
 EIGEN_DEVICE_FUNC
 EIGEN_ALWAYS_INLINE T mini(const T& x, const T& y)