[libc++] Constrain additional overloads of pow for complex harder (#110235)

Fixes #109858.

The changes in #81379 broke some 3rd party library code that expected
usability of `std::complex<NonFloatingPoint>`. Although such code isn't
portable per [complex.numbers.general]/2, it might be better to make
these additional overloads not to interfere overload resolution too
much.

---------

Co-authored-by: Louis Dionne <[email protected]>

Author: frederick-vs-ja

libcxx/include/complex

@@ -1097,20 +1097,20 @@ inline _LIBCPP_HIDE_FROM_ABI complex<_Tp> pow(const complex<_Tp>& __x, const com
   return std::exp(__y * std::log(__x));
 }
 
-template <class _Tp, class _Up>
+template <class _Tp, class _Up, __enable_if_t<is_floating_point<_Tp>::value && is_floating_point<_Up>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI complex<typename __promote<_Tp, _Up>::type>
 pow(const complex<_Tp>& __x, const complex<_Up>& __y) {
   typedef complex<typename __promote<_Tp, _Up>::type> result_type;
   return std::pow(result_type(__x), result_type(__y));
 }
 
-template <class _Tp, class _Up, __enable_if_t<is_arithmetic<_Up>::value, int> = 0>
+template <class _Tp, class _Up, __enable_if_t<is_floating_point<_Tp>::value && is_arithmetic<_Up>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI complex<typename __promote<_Tp, _Up>::type> pow(const complex<_Tp>& __x, const _Up& __y) {
   typedef complex<typename __promote<_Tp, _Up>::type> result_type;
   return std::pow(result_type(__x), result_type(__y));
 }
 
-template <class _Tp, class _Up, __enable_if_t<is_arithmetic<_Tp>::value, int> = 0>
+template <class _Tp, class _Up, __enable_if_t<is_arithmetic<_Tp>::value && is_floating_point<_Up>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI complex<typename __promote<_Tp, _Up>::type> pow(const _Tp& __x, const complex<_Up>& __y) {
   typedef complex<typename __promote<_Tp, _Up>::type> result_type;
   return std::pow(result_type(__x), result_type(__y));

libcxx/test/libcxx/numerics/complex.number/cmplx.over.pow.pass.cpp

@@ -0,0 +1,84 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <complex>
+
+//  template<class T, class U> complex<__promote<T, U>::type> pow(const complex<T>&, const U&);
+//  template<class T, class U> complex<__promote<T, U>::type> pow(const complex<T>&, const complex<U>&);
+//  template<class T, class U> complex<__promote<T, U>::type> pow(const T&, const complex<U>&);
+
+// Test that these additional overloads are free from catching std::complex<non-floating-point>,
+// which is expected by several 3rd party libraries, see https://github.com/llvm/llvm-project/issues/109858.
+//
+// Note that we reserve the right to break this in the future if we have a reason to, but for the time being,
+// make sure we don't break this property unintentionally.
+#include <cassert>
+#include <cmath>
+#include <complex>
+#include <type_traits>
+
+#include "test_macros.h"
+
+namespace usr {
+struct usr_tag {};
+
+template <class T, class U>
+typename std::enable_if<(std::is_same<T, usr_tag>::value && std::is_floating_point<U>::value) ||
+                            (std::is_floating_point<T>::value && std::is_same<U, usr_tag>::value),
+                        int>::type
+pow(const T&, const std::complex<U>&) {
+  return std::is_same<T, usr_tag>::value ? 0 : 1;
+}
+
+template <class T, class U>
+typename std::enable_if<(std::is_same<T, usr_tag>::value && std::is_floating_point<U>::value) ||
+                            (std::is_floating_point<T>::value && std::is_same<U, usr_tag>::value),
+                        int>::type
+pow(const std::complex<T>&, const U&) {
+  return std::is_same<U, usr_tag>::value ? 2 : 3;
+}
+
+template <class T, class U>
+typename std::enable_if<(std::is_same<T, usr_tag>::value && std::is_floating_point<U>::value) ||
+                            (std::is_floating_point<T>::value && std::is_same<U, usr_tag>::value),
+                        int>::type
+pow(const std::complex<T>&, const std::complex<U>&) {
+  return std::is_same<T, usr_tag>::value ? 4 : 5;
+}
+} // namespace usr
+
+int main(int, char**) {
+  using std::pow;
+  using usr::pow;
+
+  usr::usr_tag tag;
+  const std::complex<usr::usr_tag> ctag;
+
+  assert(pow(tag, std::complex<float>(1.0f)) == 0);
+  assert(pow(std::complex<float>(1.0f), tag) == 2);
+  assert(pow(tag, std::complex<double>(1.0)) == 0);
+  assert(pow(std::complex<double>(1.0), tag) == 2);
+  assert(pow(tag, std::complex<long double>(1.0l)) == 0);
+  assert(pow(std::complex<long double>(1.0l), tag) == 2);
+
+  assert(pow(1.0f, ctag) == 1);
+  assert(pow(ctag, 1.0f) == 3);
+  assert(pow(1.0, ctag) == 1);
+  assert(pow(ctag, 1.0) == 3);
+  assert(pow(1.0l, ctag) == 1);
+  assert(pow(ctag, 1.0l) == 3);
+
+  assert(pow(ctag, std::complex<float>(1.0f)) == 4);
+  assert(pow(std::complex<float>(1.0f), ctag) == 5);
+  assert(pow(ctag, std::complex<double>(1.0)) == 4);
+  assert(pow(std::complex<double>(1.0), ctag) == 5);
+  assert(pow(ctag, std::complex<long double>(1.0l)) == 4);
+  assert(pow(std::complex<long double>(1.0l), ctag) == 5);
+
+  return 0;
+}