[gcc r14-11649] libstdc++: Correct preprocessing checks for floatX_t and bfloat_16 formatting

[Tomasz Kaminski]

https://gcc.gnu.org/g:a295863e953c772a0ae91a11f652d1f641d3a3dc

commit r14-11649-ga295863e953c772a0ae91a11f652d1f641d3a3dc
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Tue Mar 11 11:59:36 2025 +0100

    libstdc++: Correct preprocessing checks for floatX_t and bfloat_16 formatting
    
    Floating points types _Float16, _Float32, _Float64, and bfloat16,
    can be formatted only if std::to_chars overloads for such types
    were provided. Currently this is only the case for architectures
    where float and double are 32-bits and 64-bits IEEE floating points types.
    
    This patch updates the preprocessing checks for formatters
    for above types to check _GLIBCXX_FLOAT_IS_IEEE_BINARY32
    and _GLIBCXX_DOUBLE_IS_IEEE_BINARY64. Making them non-formattable
    on non-IEEE architectures.
    
    Remove a potential UB, where we could produce basic_format_arg
    with _M_type set to _Arg_fp32 or _Arg_fp64, that was later not
    handled by `_M_visit`.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (formatter<_Float16, _CharT>): Define only if
            _GLIBCXX_FLOAT_IS_IEEE_BINARY32 macro is defined.
            (formatter<_Float16, _CharT>): As above.
            (formatter<__gnu_cxx::__bfloat16_t, _CharT>): As above.
            (formatter<_Float64, _CharT>): Define only if
            _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 is defined.
            (basic_format_arg::_S_to_arg_type): Normalize _Float32 and _Float64
            only to float and double respectivelly.
            (basic_format_arg::_S_to_enum): Remove handling of _Float32 and _Float64.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
    (cherry picked from commit 445128c12cf22081223f7385196ee3889ef4c4b2)

Diff:
---
 libstdc++-v3/include/std/format | 32 ++++++++------------------------
 1 file changed, 8 insertions(+), 24 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index b8830766f40f..604f4cde3c47 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2208,7 +2208,7 @@ namespace __format
     };
 #endif
 
-#ifdef __STDCPP_FLOAT16_T__
+#if defined(__STDCPP_FLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for _Float16.
   template<__format::__char _CharT>
     struct formatter<_Float16, _CharT>
@@ -2230,7 +2230,7 @@ namespace __format
     };
 #endif
 
-#if defined(__FLT32_DIG__)
+#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for _Float32.
   template<__format::__char _CharT>
     struct formatter<_Float32, _CharT>
@@ -2252,7 +2252,7 @@ namespace __format
     };
 #endif
 
-#if defined(__FLT64_DIG__)
+#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
   // Reuse __formatter_fp<C>::format<double, Out> for _Float64.
   template<__format::__char _CharT>
     struct formatter<_Float64, _CharT>
@@ -2296,7 +2296,7 @@ namespace __format
     };
 #endif
 
-#ifdef __STDCPP_BFLOAT16_T__
+#if defined(__STDCPP_BFLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for bfloat16_t.
   template<__format::__char _CharT>
     struct formatter<__gnu_cxx::__bfloat16_t, _CharT>
@@ -3367,22 +3367,16 @@ namespace __format
 	    return type_identity<float>();
 #endif
 
-#ifdef __FLT32_DIG__
+#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
 	  else if constexpr (is_same_v<_Td, _Float32>)
-# ifdef _GLIBCXX_FLOAT_IS_IEEE_BINARY32
 	    return type_identity<float>();
-# else
-	    return type_identity<_Float32>();
-# endif
 #endif
-#ifdef __FLT64_DIG__
+
+#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
 	  else if constexpr (is_same_v<_Td, _Float64>)
-# ifdef _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
 	    return type_identity<double>();
-# else
-	    return type_identity<_Float64>();
-# endif
 #endif
+
 #if _GLIBCXX_FORMAT_F128
 # if __FLT128_DIG__
 	  else if constexpr (is_same_v<_Td, _Float128>)
@@ -3462,16 +3456,6 @@ namespace __format
 	    return _Arg_u128;
 #endif
 
-	  // N.B. some of these types will never actually be used here,
-	  // because they get normalized to a standard floating-point type.
-#if defined __FLT32_DIG__ && ! _GLIBCXX_FLOAT_IS_IEEE_BINARY32
-	  else if constexpr (is_same_v<_Tp, _Float32>)
-	    return _Arg_f32;
-#endif
-#if defined __FLT64_DIG__ && ! _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
-	  else if constexpr (is_same_v<_Tp, _Float64>)
-	    return _Arg_f64;
-#endif
 #if _GLIBCXX_FORMAT_F128 == 2
 	  else if constexpr (is_same_v<_Tp, __format::__float128_t>)
 	    return _Arg_f128;