c++: Fix ICE-on-invalid with lambda template [PR94507]

While reducing something else I noticed that we ICE on the following
invalid code.  In tsubst_lambda_expr, tsubst_template_decl has already
reported an error and returned the error_mark_node, so make sure we
don't ICE on that.  I'm using a goto here because we still have to
do finish_struct because it does popclass ().

	PR c++/94507 - ICE-on-invalid with lambda template.
	* pt.c (tsubst_lambda_expr): Cope when tsubst_template_decl or
	tsubst_function_decl returns error_mark_node.

	* g++.dg/cpp2a/lambda-generic7.C: New test.

gcc/cp/ChangeLog

+2020-04-08  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94507 - ICE-on-invalid with lambda template.
+	* pt.c (tsubst_lambda_expr): Cope when tsubst_template_decl or
+	tsubst_function_decl returns error_mark_node.
+
 2020-04-08  Martin Liska  <mliska@suse.cz>
 
 	PR c++/94314

gcc/cp/pt.c

@@ -18953,6 +18953,11 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
       if (oldtmpl)
 	{
 	  tmpl = tsubst_template_decl (oldtmpl, args, complain, fntype);
+	  if (tmpl == error_mark_node)
+	    {
+	      r = error_mark_node;
+	      goto out;
+	    }
 	  fn = DECL_TEMPLATE_RESULT (tmpl);
 	  finish_member_declaration (tmpl);
 	}
@@ -18960,6 +18965,11 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	{
 	  tmpl = NULL_TREE;
 	  fn = tsubst_function_decl (oldfn, args, complain, fntype);
+	  if (fn == error_mark_node)
+	    {
+	      r = error_mark_node;
+	      goto out;
+	    }
 	  finish_member_declaration (fn);
 	}
 
@@ -19025,6 +19035,7 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
       maybe_add_lambda_conv_op (type);
     }
 
+out:
   finish_struct (type, /*attr*/NULL_TREE);
 
   insert_pending_capture_proxies ();

gcc/testsuite/ChangeLog

+2020-04-08  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94507 - ICE-on-invalid with lambda template.
+	* g++.dg/cpp2a/lambda-generic7.C: New test.
+
 2020-04-08  Alexandre Oliva <oliva@adacore.com>
 
 	* gcc.target/arm/fp16-aapcs-3.c: Explicitly use the

gcc/testsuite/g++.dg/cpp2a/lambda-generic7.C

+// PR c++/94507 - ICE-on-invalid with lambda template.
+// { dg-do compile { target c++2a } }
+
+struct S { };
+
+template<typename T, typename U>
+auto foo(T, U)
+{
+  [] <> () { foo (S{}, S{}); }; // { dg-error "expected" }
+}