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.

gcc/cp/ChangeLog

 2020-04-17  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94483
+	* lambda.c (lambda_capture_field_type): Avoid doing auto deduction if
+	the explicit initializer has parameter packs.
+
 	PR c++/88754
 	* parser.c (cp_parser_check_template_parameters): Before issuing a hard
 	error, first try simulating an error instead.

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))
     {

gcc/testsuite/ChangeLog

 2020-04-17  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94483
+	* g++.dg/cpp2a/lambda-pack-init5.C: New test.
+
 	PR c++/88754
 	* g++.dg/parse/ambig10.C: New test.
 

gcc/testsuite/g++.dg/cpp2a/lambda-pack-init5.C

+// PR c++/94483
+// { dg-do compile { target c++2a } }
+
+template<int... a> constexpr auto x1
+  = [...z = -a] (auto F) { return F(z...); };
+
+template<const int&... a> constexpr auto x2
+  = [&...z = a] (auto F) { return F(z...); };
+
+template<int... a> constexpr auto x3
+  = [z = -a] (auto F) { return F(z); }; // { dg-error "packs not expanded" }
+
+
+constexpr auto sum = [] (auto... xs) { return (xs + ... + 0); };
+const int y1 = 1, y2 = 2, y3 = 3;
+
+static_assert(x1<1,2,3>(sum) == -6);
+static_assert(x2<y1,y2,y3>(sum) == 6);