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.

Author: jwakely

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 = "";
 

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>