c++: constrained partial spec forward decl [PR96363]

Here during cp_parser_single_declaration for #2, we were calling
associate_classtype_constraints for TPL<T> (the primary template type)
before maybe_process_partial_specialization could get a chance to
notice that we're in fact declaring a distinct constrained partial
spec and not redeclaring the primary template.  This caused us to
emit a bogus error about differing constraints b/t the primary template
and #2's constraints.  This patch fixes this by moving the call to
associate_classtype_constraints after the call to shadow_tag (which
calls maybe_process_partial_specialization) and adjusting shadow_tag to
use the return value of m_p_p_s.

Moreover, if we later try to define a constrained partial specialization
that's been declared earlier (as in the third testcase), then
maybe_new_partial_specialization correctly notices it's a redeclaration
and returns NULL_TREE.  But in this case we also need to update TYPE to
point to the redeclared partial spec (it'll otherwise continue pointing
to the primary template type, eventually leading to a bogus error).

	PR c++/96363

gcc/cp/ChangeLog:

	* decl.cc (shadow_tag): Use the return value of
	maybe_process_partial_specialization.
	* parser.cc (cp_parser_single_declaration): Call shadow_tag
	before associate_classtype_constraints.
	* pt.cc (maybe_new_partial_specialization): Change return type
	to bool.  Take 'type' argument by mutable reference.  Set 'type'
	to point to the correct constrained specialization when
	appropriate.
	(maybe_process_partial_specialization): Adjust accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-partial-spec12.C: New test.
	* g++.dg/cpp2a/concepts-partial-spec12a.C: New test.
	* g++.dg/cpp2a/concepts-partial-spec13.C: New test.

(cherry picked from commit 97dc78d705a90c1ae83c78a7f2e24942cc3a6257)

3 files changed, 40 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
new file mode 100644
index 00000000000..9bb6e2d9ea6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
@@ -0,0 +1,10 @@
+// PR c++/96363
+// { dg-do compile { target c++20 } }
+
+template<class T> class TPL;
+
+template<class T> requires true  class TPL<T>;   // #1
+template<class T> requires false class TPL<T>;   // #2 error here
+
+template<class T> requires true  class TPL<T*>;  // #1
+template<class T> requires false class TPL<T*>;  // #2 error here
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12a.C
new file mode 100644
index 00000000000..6d23a28892d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12a.C
@@ -0,0 +1,14 @@
+// PR c++/96363
+// { dg-do compile { target c++20 } }
+// A version of concepts-partial-spec12.C where the primary template is
+// constrained.
+
+template<class T> concept C = true;
+
+template<C T> class TPL;
+
+template<C T> requires true  class TPL<T>;   // #1
+template<C T> requires false class TPL<T>;   // #2 error here
+
+template<C T> requires true  class TPL<T*>;  // #1
+template<C T> requires false class TPL<T*>;  // #2 error here
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec13.C b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec13.C
new file mode 100644
index 00000000000..b5ec2c92585
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec13.C
@@ -0,0 +1,16 @@
+// PR c++/99501
+// { dg-do compile { target c++20 } }
+
+template<auto> struct X;
+
+template<auto V> requires requires{V.a;} struct X<V>;
+template<auto V> requires requires{V.b;} struct X<V>;
+
+template<auto V> requires requires{V.a;} struct X<V> { static const bool v = false; };
+template<auto V> requires requires{V.b;} struct X<V> { static const bool v = true; };
+
+struct A { int a; };
+static_assert(!X<A{}>::v);
+
+struct B { int b; };
+static_assert(X<B{}>::v);