c++: don't substitute TEMPLATE_PARM_CONSTRAINTS [PR100374]

This patch makes us avoid substituting into the TEMPLATE_PARM_CONSTRAINTS
of each template parameter except as necessary for declaration matching,
like we already do for the other constituent constraints of a declaration.

This patch also improves the CA104 implementation of explicit
specialization matching of a constrained function template inside a
class template, by considering the function's combined constraints
instead of just its trailing constraints.  This allows us to correctly
handle the first three explicit specializations in concepts-spec2.C
below, but because we compare the constraints as a whole, it means we
incorrectly accept the fourth explicit specialization which writes #3's
constraints in a different way.  For complete correctness here,
determine_specialization should use tsubst_each_template_parm_constraints
and template_parameter_heads_equivalent_p.

	PR c++/100374

gcc/cp/ChangeLog:

	* pt.cc (determine_specialization): Compare overall constraints
	not just the trailing constraints.
	(tsubst_each_template_parm_constraints): Define.
	(tsubst_friend_function): Use it.
	(tsubst_friend_class): Use it.
	(tsubst_template_parm): Don't substitute TEMPLATE_PARM_CONSTRAINTS.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-spec2.C: New test.
	* g++.dg/cpp2a/concepts-template-parm11.C: New test.

(cherry picked from commit 43c013df02fdb07f9b7a5e7e6669e6d69769d451)

2 files changed, 36 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-spec2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-spec2.C
new file mode 100644
index 00000000000..4de07352c27
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-spec2.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++20 } }
+
+template<class T, int> concept C = true;
+
+template<class T> struct A {
+  template<C<sizeof(T)> U> void f(); // #1
+  template<C<0> U>         void f(); // #2
+  template<C<-1> U>        void f(); // #3
+};
+
+constexpr int n = sizeof(int);
+template<> template<C<n> U>  void A<int>::f() { } // matches #1
+template<> template<C<0> U>  void A<int>::f() { } // matches #2
+template<> template<C<-2> U> void A<int>::f() { } // no match { dg-error "match" }
+template<> template<class U> void A<int>::f() requires C<U, -1> { } // shouldn't match #3
+// { dg-error "match" "" { xfail *-*-* } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C b/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
new file mode 100644
index 00000000000..498e3c175cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
@@ -0,0 +1,20 @@
+// PR c++/100374
+// { dg-do compile { target c++20 } }
+
+template<class T, class U>
+concept C = requires { typename T; };
+
+template<class T>
+struct A {
+  template<C<typename T::value_type> U>
+  void f();
+
+  template<C<typename T::value_type> U>
+  struct B;
+};
+
+int main() {
+  A<int> a;
+  a.f<void>();
+  using type = A<int>::B<void>;
+}