c++: Non-type-dependent variadic lambda init-capture [PR94483]

In this PR, we're ICEing on a use of an 'int... a' template parameter pack as
part of the variadic lambda init-capture [...z=a].

The unexpected thing about this variadic init-capture is that it is not
type-dependent, and so the call to do_auto_deduction from
lambda_capture_field_type actually resolves its type to 'int' instead of exiting
early like it does for a type-dependent variadic initializer.  This later
confuses add_capture which, according to one of its comments, assumes that
'type' is always 'auto' for a variadic init-capture.

The simplest fix (and the approach that this patch takes) seems to be to avoid
doing auto deduction in lambda_capture_field_type when the initializer uses
parameter packs, so that we always return 'auto' even in the non-type-dependent
case.

gcc/cp/ChangeLog:

	PR c++/94483
	* lambda.c (lambda_capture_field_type): Avoid doing auto deduction if
	the explicit initializer has parameter packs.

gcc/testsuite/ChangeLog:

	PR c++/94483
	* g++.dg/cpp2a/lambda-pack-init5.C: New test.

1 files changed, 4 insertions, 1 deletions

diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
index 4f39f99..b55c2f8 100644
--- a/gcc/cp/lambda.c
+++ b/gcc/cp/lambda.c
@@ -223,7 +223,10 @@ lambda_capture_field_type (tree expr, bool explicit_init_p,
 	/* Add the reference now, so deduction doesn't lose
 	   outermost CV qualifiers of EXPR.  */
 	type = build_reference_type (type);
-      type = do_auto_deduction (type, expr, auto_node);
+      if (uses_parameter_packs (expr))
+	/* Stick with 'auto' even if the type could be deduced.  */;
+      else
+	type = do_auto_deduction (type, expr, auto_node);
     }
   else if (!is_this && type_dependent_expression_p (expr))
     {