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From: Jason Merrill <jason@gcc.gnu.org>
To: gcc-cvs@gcc.gnu.org
Subject: [gcc r12-2230] c++: concepts TS and explicit specialization [PR101098]
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https://gcc.gnu.org/g:ddd25bd1a7c8f456bc914e34b77d43f39a1062d4

commit r12-2230-gddd25bd1a7c8f456bc914e34b77d43f39a1062d4
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Jul 9 13:50:01 2021 -0400

    c++: concepts TS and explicit specialization [PR101098]
    
    duplicate_decls was not recognizing the explicit specialization as matching
    the implicit specialization of g<Y> because
    function_requirements_equivalent_p was seeing the C constraint on the
    implicit one and not on the explicit.
    
            PR c++/101098
    
    gcc/cp/ChangeLog:
    
            * decl.c (function_requirements_equivalent_p): Only compare
            trailing requirements on a specialization.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/concepts/explicit-spec1.C: New test.

Diff:
---
 gcc/cp/decl.c                                  | 4 +++-
 gcc/testsuite/g++.dg/concepts/explicit-spec1.C | 9 +++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index ebe1318d38d..0df689b01f8 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -955,7 +955,9 @@ static bool
 function_requirements_equivalent_p (tree newfn, tree oldfn)
 {
   /* In the concepts TS, the combined constraints are compared.  */
-  if (cxx_dialect < cxx20)
+  if (cxx_dialect < cxx20
+      && (DECL_TEMPLATE_SPECIALIZATION (newfn)
+	  <= DECL_TEMPLATE_SPECIALIZATION (oldfn)))
     {
       tree ci1 = get_constraints (oldfn);
       tree ci2 = get_constraints (newfn);
diff --git a/gcc/testsuite/g++.dg/concepts/explicit-spec1.C b/gcc/testsuite/g++.dg/concepts/explicit-spec1.C
new file mode 100644
index 00000000000..d9b6b3d1741
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/explicit-spec1.C
@@ -0,0 +1,9 @@
+// PR c++/101098
+// { dg-do compile { target concepts } }
+
+template<typename T> concept C = __is_class(T);
+struct Y { int n; } y;
+template<C T> void g(T) { }
+int called;
+template<> void g(Y) { called = 3; }
+int main() { g(y); }