libstdc++: Fix std::format_kind primary template for Clang [PR120190]

Although Clang trunk has been adjusted to handle our std::format_kind
definition (because they need to be able to compile the GCC 15.1.0
release), it's probably better to not rely on something that they might
start diagnosing again in future.

Define the primary template in terms of an immediately invoked function
expression, so that we can put a static_assert(false) in the body.

libstdc++-v3/ChangeLog:

	PR libstdc++/120190
	* include/std/format (format_kind): Adjust primary template to
	not depend on itself.
	* testsuite/std/format/ranges/format_kind_neg.cc: Adjust
	expected errors. Check more invalid specializations.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Daniel Krügler <daniel.kruegler@gmail.com>

2 files changed, 24 insertions, 10 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index bfda589..b1823db 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -5464,13 +5464,22 @@ namespace __format
     debug_string
   };
 
-  /// @cond undocumented
+  /** @brief A constant determining how a range should be formatted.
+   *
+   * The primary template of `std::format_kind` cannot be instantiated.
+   * There is a partial specialization for input ranges and you can
+   * specialize the variable template for your own cv-unqualified types
+   * that satisfy the `ranges::input_range` concept.
+   *
+   * @since C++23
+   */
   template<typename _Rg>
-    constexpr auto format_kind =
-    __primary_template_not_defined(
-      format_kind<_Rg> // you can specialize this for non-const input ranges
-    );
+    constexpr auto format_kind = []{
+      static_assert(false, "cannot use primary template of 'std::format_kind'");
+      return type_identity<_Rg>{};
+    }();
 
+  /// @cond undocumented
   template<typename _Tp>
     consteval range_format
     __fmt_kind()
diff --git a/libstdc++-v3/testsuite/std/format/ranges/format_kind_neg.cc b/libstdc++-v3/testsuite/std/format/ranges/format_kind_neg.cc
index bf8619d..0d761ae 100644
--- a/libstdc++-v3/testsuite/std/format/ranges/format_kind_neg.cc
+++ b/libstdc++-v3/testsuite/std/format/ranges/format_kind_neg.cc
@@ -5,9 +5,14 @@
 
 #include <format>
 
-template<auto> struct Tester { };
+void test()
+{
+  (void) std::format_kind<void>; // { dg-error "here" }
+  (void) std::format_kind<const void>; // { dg-error "here" }
+  (void) std::format_kind<int>; // { dg-error "here" }
+  (void) std::format_kind<int&>; // { dg-error "here" }
+  (void) std::format_kind<const int(&)[10]>; // { dg-error "here" }
+  (void) std::format_kind<void()>; // { dg-error "here" }
+}
 
-Tester<std::format_kind<const int(&)[1]>> t; // { dg-error "here" }
-
-// { dg-error "use of 'std::format_kind" "" { target *-*-* } 0 }
-// { dg-error "primary_template_not_defined" "" { target *-*-* } 0 }
+// { dg-error "cannot use primary template of 'std::format_kind'" "" { target *-*-* } 0 }