c++: find_template_parameters and TEMPLATE_DECLs [PR101247]

r12-1989 fixed the testcase in the PR, but unfortunately the fix is
buggy: it breaks the case where the common template between the
TEMPLATE_DECL t and ctx_parms is the innermost template (as in
concepts-memtmpl5.C below).  This can be fixed by instead passing the
TREE_TYPE of ctmpl to common_enclosing_class when ctmpl is a class
template.

But even after that's fixed, the analogous case where the innermost
template is a partial specialization is still broken (as in
concepts-memtmpl5a.C below), because ctmpl is always a primary template.

So this patch instead takes a diferent approach that doesn't rely on
ctx_parms at all: when looking for the template parameters of a
TEMPLATE_DECL that are shared with the current template context, just
walk its DECL_CONTEXT.  As long as the template is not overly general
(e.g. we didn't pass it through most_general_template), this should give
us exactly what we want, since if a TEMPLATE_DECL can be referred to
from some template context then the template parameters it uses must all
be in-scope and contained in its DECL_CONTEXT.  This effectively makes
us treat TEMPLATE_DECLs more similarly to other _DECLs (whose DECL_CONTEXT
we also walk).

	PR c++/101247

gcc/cp/ChangeLog:

	* pt.c (any_template_parm_r) <case TEMPLATE_DECL>: Just walk the
	DECL_CONTEXT.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-memtmpl4.C: Uncomment the commented out
	example, which we now handle correctly.
	* g++.dg/cpp2a/concepts-memtmpl5.C: New test.
	* g++.dg/cpp2a/concepts-memtmpl5a.C: New test.

4 files changed, 32 insertions, 10 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 7e56ccf..dc0f0b7 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10728,15 +10728,11 @@ any_template_parm_r (tree t, void *data)
       break;
 
     case TEMPLATE_DECL:
-      {
-	/* If T is a member template that shares template parameters with
-	   ctx_parms, we need to mark all those parameters for mapping.  */
-	if (tree ctmpl = TREE_TYPE (INNERMOST_TEMPLATE_PARMS (ftpi->ctx_parms)))
-	  if (tree com = common_enclosing_class (DECL_CONTEXT (t),
-						 DECL_CONTEXT (ctmpl)))
-	    if (tree ti = CLASSTYPE_TEMPLATE_INFO (com))
-	      WALK_SUBTREE (TI_ARGS (ti));
-      }
+      /* If T is a member template that shares template parameters with
+	 ctx_parms, we need to mark all those parameters for mapping.
+	 To that end, it should suffice to just walk the DECL_CONTEXT of
+	 the template (assuming the template is not overly general).  */
+      WALK_SUBTREE (DECL_CONTEXT (t));
       break;
 
     case LAMBDA_EXPR:
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl4.C
index 625149e..f990ae1 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl4.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl4.C
@@ -24,5 +24,5 @@ int main()
 {
   A<void>::B::f(0);
   A<void>::C<int>::f(0);
-  // A<void>::C<int>::g();
+  A<void>::C<int>::g();
 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5.C
new file mode 100644
index 0000000..3c83bb8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5.C
@@ -0,0 +1,11 @@
+// PR c++/101247
+// { dg-do compile { target concepts } }
+
+template<class T, class U> struct A {
+  template<class> static constexpr bool d = true;
+  static void g() requires d<U>;
+};
+
+int main() {
+  A<int, char>::g();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5a.C
new file mode 100644
index 0000000..458f1cd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-memtmpl5a.C
@@ -0,0 +1,15 @@
+// PR c++/101247
+// { dg-do compile { target concepts } }
+// A variant of concepts-memtmpl5.C that uses a partial specialization
+// of A instead of the primary template.
+
+template<class, class> struct A;
+
+template<class T, class U> requires true struct A<T, U> {
+  template<class V> static constexpr bool d = true;
+  static void g() requires d<U>;
+};
+
+int main() {
+  A<int, char>::g();
+}