libstdc++: Do not specialize std::formatter for incomplete type [PR120625]

Using an incomplete type as the template argument for std::formatter
specializations causes problems for program-defined specializations of
std::formatter which have constraints. When the compiler has to find
which specialization of std::formatter to use for the incomplete type it
considers the program-defined specializations and checks to see if their
constraints are satisfied, which can give errors if the constraints
cannot be checked for incomplete types.

This replaces the base class of the disabled specializations with a
concrete class __formatter_disabled, so there is no need to match a
specialization and no more incomplete type.

libstdc++-v3/ChangeLog:

	PR libstdc++/120625
	* include/std/format (__format::__disabled): Remove.
	(__formatter_disabled): New type.
	(formatter<char*, wchar_t>, formatter<const char*, wchar_t>)
	(formatter<char[N], wchar_t>, formatter<string, wchar_t>)
	(formatter<string_view, wchar_t>): Use __formatter_disabled as
	base class instead of formatter<__disabled, wchar_t>.
	* testsuite/std/format/formatter/120625.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/format/formatter/120625.cc b/libstdc++-v3/testsuite/std/format/formatter/120625.cc
new file mode 100644
index 0000000..6b03af9
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/format/formatter/120625.cc
@@ -0,0 +1,19 @@
+// { dg-do compile { target c++20 } }
+
+// Bug libstdc++/120625
+// std::formatter<__disabled> specializations cause errors in user code
+
+#include <format>
+
+enum X { };
+
+// A concept that cannot be used with incomplete types:
+template<typename T>
+concept is_X = !std::is_empty_v<T> && std::is_same_v<X, T>;
+
+// A valid program-defined specialization:
+template<typename T, typename C> requires is_X<T>
+struct std::formatter<T, C> : std::formatter<int, C> { };
+
+// Instantiate the program-defined formatter specialization:
+auto s = sizeof(std::formatter<X, char>);