c++: constrained auto in lambda using outer tparms [PR103706]

Here we're crashing during satisfaction of the lambda's placeholder type
constraints because the constraints depend on the template arguments
from the enclosing scope, which aren't part of the lambda's DECL_TI_ARGS.

This patch fixes this by making do_auto_deduction consider the
"regenerating" template arguments of a lambda for satisfaction,
mirroring what's done in satisfy_declaration_constraints.

	PR c++/103706

gcc/cp/ChangeLog:

	* constraint.cc (satisfy_declaration_constraints): Use
	lambda_regenerating_args instead.
	* cp-tree.h (lambda_regenerating_args): Declare.
	* pt.cc (lambda_regenerating_args): Define, split out from
	satisfy_declaration_constraints.
	(do_auto_deduction): Use lambda_regenerating_args to obtain the
	full set of outer template arguments for satisfaction when
	inside a lambda.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda18.C: New test.

1 files changed, 3 insertions, 4 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index cc4df42..b7b9439 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3154,12 +3154,11 @@ satisfy_declaration_constraints (tree t, sat_info info)
 	 set of template arguments.  Augment this with the outer template
 	 arguments that were used to regenerate the lambda.  */
       gcc_assert (!args || TMPL_ARGS_DEPTH (args) == 1);
-      tree lambda = CLASSTYPE_LAMBDA_EXPR (DECL_CONTEXT (t));
-      tree outer_args = TI_ARGS (LAMBDA_EXPR_REGEN_INFO (lambda));
+      tree regen_args = lambda_regenerating_args (t);
       if (args)
-	args = add_to_template_args (outer_args, args);
+	args = add_to_template_args (regen_args, args);
       else
-	args = outer_args;
+	args = regen_args;
     }
 
   /* If any arguments depend on template parameters, we can't