c++: templated substitution into lambda-expr, cont [PR114393]

The original PR114393 testcase is unfortunately still not accepted after
r14-9938-g081c1e93d56d35 due to return type deduction confusion when a
lambda-expr is used as a default template argument.

The below reduced testcase demonstrates the bug.  Here when forming the
dependent specialization b_v<U> we substitute the default argument of F,
a lambda-expr, with _Descriptor=U.  (In this case in_template_context is
true since we're in the context of the template c_v, so we don't defer.)
This substitution in turn lowers the level of the lambda's auto return
type from 2 to 1 and so later, when instantiating c_v<int, char> we wrongly
substitute this auto with the template argument at level=0,index=0, i.e.
int, instead of going through do_auto_deduction which would yield char.

One way to fix this would be to use a level-less auto to represent a
deduced return type of a lambda, but that might be too invasive of a
change at this stage, and it might be better to do this across the board
for all deduced return types.

Another way would be to pass tf_partial from coerce_template_parms during
dependent substitution into a default template argument so that the
substitution doesn't do any level-lowering, but that wouldn't do the right
thing in this case due to the tf_partial early exit in the LAMBDA_EXPR
case of tsubst_expr.

Yet another way, and the approach that this patch takes, is to just
defer all dependent substitution into a lambda-expr, building upon the
logic added in r14-9938-g081c1e93d56d35.  This also helps ensure
LAMBDA_EXPR_REGEN_INFO consists only of the concrete template arguments
that were ultimately substituted into the most general lambda.

	PR c++/114393

gcc/cp/ChangeLog:

	* pt.cc (tsubst_lambda_expr): Also defer all dependent
	substitution.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ2a.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

2 files changed, 21 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index ec259ee..3b2106d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -19622,11 +19622,16 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
   in_decl = oldfn;
 
   args = add_extra_args (LAMBDA_EXPR_EXTRA_ARGS (t), args, complain, in_decl);
-  if (processing_template_decl && !in_template_context)
+  if (processing_template_decl
+      && (!in_template_context || any_dependent_template_arguments_p (args)))
     {
       /* Defer templated substitution into a lambda-expr if we lost the
 	 necessary template context.  This may happen for a lambda-expr
-	 used as a default template argument.  */
+	 used as a default template argument.
+
+	 Defer dependent substitution as well so that we don't prematurely
+	 lower the level of a deduced return type or any other auto or
+	 template parameter belonging to the lambda.  */
       t = copy_node (t);
       LAMBDA_EXPR_EXTRA_ARGS (t) = NULL_TREE;
       LAMBDA_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, complain);
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
new file mode 100644
index 0000000..7136ce7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C
@@ -0,0 +1,14 @@
+// PR c++/114393
+// { dg-do compile { target c++20 } }
+
+template <auto _DescriptorFn> struct c1 {};
+
+template <class _Descriptor, auto F = [] { return _Descriptor(); }>
+inline constexpr auto b_v = F;
+
+template <class T, class U>
+inline constexpr auto c_v = b_v<U>;
+
+auto f = c_v<int, char>;
+using type = decltype(f());
+using type = char;