[gcc r13-9549] libstdc++: Correct preprocessing checks for floatX_t and bfloat_16 formatting

[Tomasz Kaminski]

https://gcc.gnu.org/g:5e23f9be635c1db34d494e1c4f980fd224406af4

commit r13-9549-g5e23f9be635c1db34d494e1c4f980fd224406af4
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.
    
    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 (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.
    
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
    
            (cherry picked with modifications from commit 445128c12cf22081223f7385196ee3889ef4c4b2)

Diff:
---
 libstdc++-v3/include/std/format | 24 ++++--------------------
 1 file changed, 4 insertions(+), 20 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index f46012a2b014..c7e43330e7e6 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2979,22 +2979,16 @@ namespace __format
 
 	  // TODO bfloat16 and float16
 
-#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>)
@@ -3074,16 +3068,6 @@ 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;