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

[Jason Merrill]

On 3/12/19 3:59 PM, Marek Polacek wrote:
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  <pola...@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");

That diagnostic seems a bit obscure, how about something like "too many 
%<...%> in lambda capture"?

Or perhaps check to see if there's an initializer after the ..., and 
complain about the second ... if so, or the first ... if not.

Jason