[gcc r11-6154] libstdc++: Warn if __STRICT_ANSI has been undefined

[gcc r11-6154] libstdc++: Warn if __STRICT_ANSI has been undefined

[Jonathan Wakely]

https://gcc.gnu.org/g:767537a8b027bcb5807bb45b0268c5da98c2c7a0

commit r11-6154-g767537a8b027bcb5807bb45b0268c5da98c2c7a0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 16 15:54:50 2020 +0000

    libstdc++: Warn if __STRICT_ANSI has been undefined
    
    Recent changes to use __int128 as an integer-like type in <ranges> and
    to optimize std::uniform_int_distribution mean that the library relies
    on __int128 more heavily than in the past.
    
    The library expects that if __int128 is supported then either
    __GLIBCXX_TYPE_INT_N_0 is defined (and we treat is like the standard
    integer types), or __STRICT_ANSI__ is defined (and we need to add
    special handling for __int128 as a non-standard integer type).
    
    If users compile with -std=c++NN -U__STRICT_ANSI__ then it puts the
    library into a broken and inconsistent state, where the compiler doesn't
    define the __GLIBCXX_TYPE_INT_N_0 macro, but the library thinks it
    doesn't need special handling for __int128. What the user should do is
    compile with -std=gnu++NN instead.
    
    This adds a warning if it appears that __int128 is supported but neither
    __GLIBCXX_TYPE_INT_N_0 nor __STRICT_ANSI__ is defined.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/c++config: Warn if __STRICT_ANSI__ state is
            inconsistent with __GLIBCXX_TYPE_INT_N_0.

Diff:
---
 libstdc++-v3/include/bits/c++config | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 155d0f46b16..cd7678eb16d 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -524,6 +524,15 @@ namespace std
 
 #define _GLIBCXX_USE_ALLOCATOR_NEW
 
+#ifdef __SIZEOF_INT128__
+#if ! defined __GLIBCXX_TYPE_INT_N_0 && ! defined __STRICT_ANSI__
+// If __int128 is supported, we expect __GLIBCXX_TYPE_INT_N_0 to be defined
+// unless the compiler is in strict mode. If it's not defined and the strict
+// macro is not defined, something is wrong.
+#warning "__STRICT_ANSI__ seems to have been undefined; this is not supported"
+#endif
+#endif
+
 #else // !__cplusplus
 # define _GLIBCXX_BEGIN_EXTERN_C
 # define _GLIBCXX_END_EXTERN_C