[gcc r12-2996] libstdc++: Simplify n-ary arithmetic promotion traits

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:37620d575111caf3b24d4b9039808334e619c13a

commit r12-2996-g37620d575111caf3b24d4b9039808334e619c13a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Aug 17 20:26:52 2021 +0100

    libstdc++: Simplify n-ary arithmetic promotion traits
    
    The std::complex partial specializations have been unnecessary since
    774c3d8647cc7012937cfc9d2d6dacc85b6cf8e9
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/ext/type_traits.h (__promote_2, __promote_3)
            (__promote_4): Redfine as alias templates using __promoted_t.
            * include/std/complex (__promote_2): Remove partial
            specializations for std::complex.

Diff:
---
 libstdc++-v3/include/ext/type_traits.h | 16 +++++++++++++++-
 libstdc++-v3/include/std/complex       | 29 -----------------------------
 2 files changed, 15 insertions(+), 30 deletions(-)

diff --git a/libstdc++-v3/include/ext/type_traits.h b/libstdc++-v3/include/ext/type_traits.h
index 065edb4e9a5..fed78d3a527 100644
--- a/libstdc++-v3/include/ext/type_traits.h
+++ b/libstdc++-v3/include/ext/type_traits.h
@@ -189,9 +189,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { typedef float __type; };
 
 #if __cpp_fold_expressions
+
   template<typename... _Tp>
     using __promoted_t = decltype((typename __promote<_Tp>::__type(0) + ...));
-#endif
+
+  // Deducing the promoted type is done by __promoted_t<_Tp...>,
+  // then __promote is used to provide the nested __type member.
+  template<typename _Tp, typename _Up>
+    using __promote_2 = __promote<__promoted_t<_Tp, _Up>>;
+
+  template<typename _Tp, typename _Up, typename _Vp>
+    using __promote_3 = __promote<__promoted_t<_Tp, _Up, _Vp>>;
+
+  template<typename _Tp, typename _Up, typename _Vp, typename _Wp>
+    using __promote_4 = __promote<__promoted_t<_Tp, _Up, _Vp, _Wp>>;
+
+#else
 
   template<typename _Tp, typename _Up,
            typename _Tp2 = typename __promote<_Tp>::__type,
@@ -219,6 +232,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       typedef __typeof__(_Tp2() + _Up2() + _Vp2() + _Wp2()) __type;
     };
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
diff --git a/libstdc++-v3/include/std/complex b/libstdc++-v3/include/std/complex
index c2f6421e0b7..a5b4406dd68 100644
--- a/libstdc++-v3/include/std/complex
+++ b/libstdc++-v3/include/std/complex
@@ -1557,35 +1557,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
-{
-_GLIBCXX_BEGIN_NAMESPACE_VERSION
-
-  // See ext/type_traits.h for the primary template.
-  template<typename _Tp, typename _Up>
-    struct __promote_2<std::complex<_Tp>, _Up>
-    {
-    public:
-      typedef std::complex<typename __promote_2<_Tp, _Up>::__type> __type;
-    };
-
-  template<typename _Tp, typename _Up>
-    struct __promote_2<_Tp, std::complex<_Up> >
-    {
-    public:
-      typedef std::complex<typename __promote_2<_Tp, _Up>::__type> __type;
-    };
-
-  template<typename _Tp, typename _Up>
-    struct __promote_2<std::complex<_Tp>, std::complex<_Up> >
-    {
-    public:
-      typedef std::complex<typename __promote_2<_Tp, _Up>::__type> __type;
-    };
-
-_GLIBCXX_END_NAMESPACE_VERSION
-} // namespace
-
 #if __cplusplus >= 201103L
 
 namespace std _GLIBCXX_VISIBILITY(default)