[gcc r14-9761] libstdc++: Guard uses of char8_t with __cpp_char8_t [PR114519]

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

https://gcc.gnu.org/g:21aa57e73b820395b7b01843d61ef5b84cd20d02

commit r14-9761-g21aa57e73b820395b7b01843d61ef5b84cd20d02
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 28 21:29:17 2024 +0000

    libstdc++: Guard uses of char8_t with __cpp_char8_t [PR114519]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/114519
            * include/bits/unicode.h (_Utf8_view): Guard with check for
            char8_t being enabled.
            (__literal_encoding_is_unicode): Guard use of char8_t with check
            for it being enabled.
            * testsuite/std/format/functions/114519.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/unicode.h                   | 10 +++++++---
 libstdc++-v3/testsuite/std/format/functions/114519.cc |  3 +++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/bits/unicode.h b/libstdc++-v3/include/bits/unicode.h
index 51bf02e927f..0e95c86a0b0 100644
--- a/libstdc++-v3/include/bits/unicode.h
+++ b/libstdc++-v3/include/bits/unicode.h
@@ -578,8 +578,10 @@ namespace __unicode
       constexpr bool empty() const { return ranges::empty(_M_base); }
     };
 
+#ifdef __cpp_char8_t
   template<typename _View>
     using _Utf8_view = _Utf_view<char8_t, _View>;
+#endif
   template<typename _View>
     using _Utf16_view = _Utf_view<char16_t, _View>;
   template<typename _View>
@@ -991,12 +993,14 @@ inline namespace __v15_1_0
     consteval bool
     __literal_encoding_is_unicode()
     {
-      if constexpr (is_same_v<_CharT, char8_t>)
-	return true;
-      else if constexpr (is_same_v<_CharT, char16_t>)
+      if constexpr (is_same_v<_CharT, char16_t>)
 	return true;
       else if constexpr (is_same_v<_CharT, char32_t>)
 	  return true;
+#ifdef __cpp_char8_t
+      else if constexpr (is_same_v<_CharT, char8_t>)
+	return true;
+#endif
 
       const char* __enc = "";
 
diff --git a/libstdc++-v3/testsuite/std/format/functions/114519.cc b/libstdc++-v3/testsuite/std/format/functions/114519.cc
new file mode 100644
index 00000000000..25a112a954e
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/format/functions/114519.cc
@@ -0,0 +1,3 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-fno-char8_t" }
+#include <format>