c++: constrained CTAD for nested class template [PR97679]

In the testcase below, we're crashing during constraint checking of the
implicitly generated deduction guides for the nested class template A::B
because we never substitute the outer template arguments (for A) into
the constraint, neither ahead of time nor as part of satisfaction.

Ideally we'd like to avoid substituting into a constraint ahead of
time, but the "flattening" vector 'tsubst_args' is constructed under the
assumption that all outer template arguments are already substituted in,
and eliminating this assumption to yield a flattening vector that
includes outer (generic) template arguments suitable for substituting
into the constraint would be tricky and error-prone.  So this patch
takes the approximate approach of substituting the outer arguments into
the constraint ahead of time, so that the subsequent substitution of
'tsubst_args' is coherent and so later satisfaction just works.

gcc/cp/ChangeLog:

	PR c++/97679
	* pt.c (build_deduction_guide): Document OUTER_ARGS.  Substitute
	them into the propagated constraints.

gcc/testsuite/ChangeLog:

	PR c++/97679
	* g++.dg/cpp2a/concepts-ctad3.C: New test.

2 files changed, 33 insertions, 3 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 46b237f..842a58c 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28602,7 +28602,8 @@ rewrite_tparm_list (tree oldelt, unsigned index, unsigned level,
 /* Returns a C++17 class deduction guide template based on the constructor
    CTOR.  As a special case, CTOR can be a RECORD_TYPE for an implicit default
    guide, REFERENCE_TYPE for an implicit copy/move guide, or TREE_LIST for an
-   aggregate initialization guide.  */
+   aggregate initialization guide.  OUTER_ARGS are the template arguments
+   for the enclosing scope of the class.  */
 
 static tree
 build_deduction_guide (tree type, tree ctor, tree outer_args, tsubst_flags_t complain)
@@ -28728,7 +28729,15 @@ build_deduction_guide (tree type, tree ctor, tree outer_args, tsubst_flags_t com
 	  if (fparms == error_mark_node)
 	    ok = false;
 	  if (ci)
-	    ci = tsubst_constraint_info (ci, tsubst_args, complain, ctor);
+	    {
+	      if (outer_args)
+		/* FIXME: We'd like to avoid substituting outer template
+		   arguments into the constraint ahead of time, but the
+		   construction of tsubst_args assumes that outer arguments
+		   are already substituted in.  */
+		ci = tsubst_constraint_info (ci, outer_args, complain, ctor);
+	      ci = tsubst_constraint_info (ci, tsubst_args, complain, ctor);
+	    }
 
 	  /* Parms are to have DECL_CHAIN tsubsted, which would be skipped if
 	     cp_unevaluated_operand.  */
@@ -28744,7 +28753,12 @@ build_deduction_guide (tree type, tree ctor, tree outer_args, tsubst_flags_t com
 	  fparms = tsubst_arg_types (fparms, targs, NULL_TREE, complain, ctor);
 	  fargs = tsubst (fargs, targs, complain, ctor);
 	  if (ci)
-	    ci = tsubst_constraint_info (ci, targs, complain, ctor);
+	    {
+	      if (outer_args)
+		/* FIXME: As above.  */
+		ci = tsubst_constraint_info (ci, outer_args, complain, ctor);
+	      ci = tsubst_constraint_info (ci, targs, complain, ctor);
+	    }
 	}
 
       --processing_template_decl;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ctad3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad3.C
new file mode 100644
index 0000000..3546b74
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad3.C
@@ -0,0 +1,16 @@
+// PR c++/97679
+// { dg-do compile { target c++20 } }
+
+template <bool V> struct A {
+  template <class T> struct B {
+    B(T) requires V;
+    template <class U> B(T, U) requires V || (__is_same(T, char) && __is_same(U, int));
+  };
+};
+
+A<true>::B x1(0);
+A<false>::B x2(0); // { dg-error "deduction|no match" }
+
+A<true>::B y1(0, '0');
+A<false>::B y2(0, '0'); // { dg-error "deduction|no match" }
+A<false>::B y3('0', 0);