[PATCH] c++: partial spec constraint checking context [PR105220]

[Patrick Palka <ppalka@redhat.com>]

Currently when checking the constraints of a class template, we do so in
the context of the template, not the specialized type.  This is the best
we can do for a primary template since the specialized type is valid
only if the primary template's constraints are satisfied.  But for a
partial specialization, we can assume the specialized type is valid (as
a consequence of constraints being checked only when necessary), so we
arguably should check the constraints on a partial specialization more
specifically in the context of the specialized type, not the template.

This patch implements this by substituting and setting the access
context appropriately in satisfy_declaration_constraints.  Note that
setting the access context in this case is somewhat redundant since the
relevant caller most_specialized_partial_spec will already have set the
access context to the specialiation, but this redundancy should be harmless.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and perhaps 12.2 (after the branch is thawed)?

	PR c++/105220

gcc/cp/ChangeLog:

	* constraint.cc (satisfy_declaration_constraints): When checking
	the constraints of a partial template specialization, do so in
	the context of the specialized type not the template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-partial-spec12.C: New test.
---
 gcc/cp/constraint.cc                          | 17 ++++++++++++++---
 .../g++.dg/cpp2a/concepts-partial-spec12.C    | 19 +++++++++++++++++++
 2 files changed, 33 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 94f6222b436..772f8532b47 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3253,11 +3253,22 @@ satisfy_declaration_constraints (tree t, tree args, sat_info info)
     {
       if (!push_tinst_level (t, args))
 	return result;
-      tree pattern = DECL_TEMPLATE_RESULT (t);
+      tree ascope = DECL_TEMPLATE_RESULT (t);
+      if (CLASS_TYPE_P (TREE_TYPE (t))
+	  && CLASSTYPE_TEMPLATE_SPECIALIZATION (TREE_TYPE (t)))
+	{
+	  gcc_checking_assert (t == most_general_template (t));
+	  /* When checking the constraints on a partial specialization,
+	     do so in the context of the specialized type, not the template.
+	     This substitution should always succeed since we shouldn't
+	     be checking constraints thereof unless the specialized type
+	     is valid.  */
+	  ascope = tsubst (ascope, args, tf_none, info.in_decl);
+	}
       push_to_top_level ();
-      push_access_scope (pattern);
+      push_access_scope (ascope);
       result = satisfy_normalized_constraints (norm, args, info);
-      pop_access_scope (pattern);
+      pop_access_scope (ascope);
       pop_from_top_level ();
       pop_tinst_level ();
     }
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..641d456722d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
@@ -0,0 +1,19 @@
+// PR c++/105220
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept fooable = requires(T t) { t.foo(); };
+
+template<class>
+struct A;        // #1, incomplete
+
+template<fooable T>
+struct A<T> { }; // #2
+
+struct B {
+private:
+  friend struct A<B>;
+  void foo();
+};
+
+template struct A<B>; // OK, B::foo() is accessible from #2
-- 
2.36.0.44.g0f828332d5