[gcc r16-658] libstdc++: Fix std::format_kind primary template for Clang [PR120190]

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:c65725eccbabf3b9b5965f27fff2d3b9f6c75930

commit r16-658-gc65725eccbabf3b9b5965f27fff2d3b9f6c75930
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu May 15 11:01:05 2025 +0100

    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 <tkami...@redhat.com>
    Reviewed-by: Daniel Krügler <daniel.krueg...@gmail.com>

Diff:
---
 libstdc++-v3/include/std/format                       | 19 ++++++++++++++-----
 .../testsuite/std/format/ranges/format_kind_neg.cc    | 15 ++++++++++-----
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index bfda5895e0c0..b1823db83bc9 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 bf8619d3d276..0d761ae11185 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 }