[gcc r16-2056] libstdc++: Format __float128 as _Float128 only when long double is not 128 IEEE [PR120976]

[Tomasz Kaminski]

https://gcc.gnu.org/g:2a82d4c859bd0eca4fe31fc79d234abd05e6a9d8

commit r16-2056-g2a82d4c859bd0eca4fe31fc79d234abd05e6a9d8
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Fri May 16 07:12:36 2025 +0200

    libstdc++: Format __float128 as _Float128 only when long double is not 128 IEEE [PR120976]
    
    For powerpc64 and sparc architectures that both have __float128 and 128bit long double,
    the __float128 is same type as long double/__ieee128 and already formattable.
    
    The remaining specialization makes __float128 formattable on x86_64 via _Float128,
    however __float128 is now not formattable on x86_32 (-m32) with -mlong-double-128,
    where __float128 is distinct type from long double that is 128bit IEEE.
    
            PR libstdc++/120976
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (formatter<__float128, _Char_T): Define if
            _GLIBCXX_FORMAT_F128 == 2.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/std/format | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 46bd5d5ee6a0..5749aa1995a9 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2998,11 +2998,9 @@ namespace __format
     };
 #endif
 
-#if defined(__SIZEOF_FLOAT128__) && _GLIBCXX_FORMAT_F128 > 1
-  // Reuse __formatter_fp<C>::format<__format::__flt128_t, Out> for __float128.
-  // This formatter is not declared if _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT is true,
-  // as __float128 when present is same type as __ieee128, which may be same as
-  // long double.
+#if defined(__SIZEOF_FLOAT128__) && _GLIBCXX_FORMAT_F128 == 2
+  // Use __formatter_fp<C>::format<__format::__flt128_t, Out> for __float128,
+  // when long double is not 128bit IEEE type.
   template<__format::__char _CharT>
     struct formatter<__float128, _CharT>
     {
@@ -3020,9 +3018,6 @@ namespace __format
 
     private:
       __format::__formatter_fp<_CharT> _M_f;
-
-      static_assert( !is_same_v<__float128, long double>,
-		     "This specialization should not be used for long double" );
     };
 #endif