C++ PATCH for c++/89686 - mixing init-capture and simple-capture in lambda

[Marek Polacek <polacek at redhat dot com>]

As Barry explained in the PR, lambda capture is one of

  simple-capture ...[opt]
  ...[opt] init-capture

where init-capture requires an initializer.  Here we have

  [...xs...]

which is ill-formed as it's mingling both of these.

Bootstrapped/regtested on x86_64-linux, ok for trunk?  Or should I defer to GCC
10?

2019-03-12  Marek Polacek  <polacek@redhat.com>

	PR c++/89686 - mixing init-capture and simple-capture in lambda.
	* parser.c (cp_parser_lambda_introducer): Give error when combining
	init-capture and simple-capture.

	* g++.dg/cpp2a/lambda-pack-init2.C: New test.

diff --git gcc/cp/parser.c gcc/cp/parser.c
index f95111169ed..d5d8f364752 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -10721,6 +10721,15 @@ cp_parser_lambda_introducer (cp_parser* parser, tree lambda_expr)
 	    {
 	      cp_lexer_consume_token (parser->lexer);
 	      capture_init_expr = make_pack_expansion (capture_init_expr);
+	      if (init_pack_expansion)
+		{
+		  /* We'd already seen '...' so we were expecting an
+		     init-capture.  But we just saw another '...' which
+		     would imply a simple-capture.  */
+		  error_at (capture_token->location,
+			    "combining init-capture and simple-capture");
+		  continue;
+		}
 	    }
 	}
 
diff --git gcc/testsuite/g++.dg/cpp2a/lambda-pack-init2.C gcc/testsuite/g++.dg/cpp2a/lambda-pack-init2.C
new file mode 100644
index 00000000000..a6172076400
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/lambda-pack-init2.C
@@ -0,0 +1,19 @@
+// PR c++/89686
+// { dg-do compile { target c++2a } }
+
+template <typename... Ts>
+void foo(Ts... xs)
+{
+  int i = 10;
+  [...xs...]{}(); // { dg-error "combining init-capture and simple-capture" }
+  [xs...]{}();
+  [...xs=xs]{}();
+  [i, ...xs...]{}(); // { dg-error "combining init-capture and simple-capture" }
+  [i, xs...]{}();
+  [i, ...xs=xs]{}();
+}
+
+int main()
+{
+ foo(0, 1, 2);
+}