[committed] c++: Fix C++20 variadic lambda init-capture grammar.

[Jason Merrill]

The grammar for variadic init-capture was fixed at the Prague C++ meeting
where we finalized C++20.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog
2020-02-24  Jason Merrill  <ja...@redhat.com>

        P0780R2: Resolve lambda init-capture pack grammar.
        * parser.c (cp_parser_lambda_introducer): Expect &...x=y rather than
        ...&x=y.
---
 gcc/cp/parser.c                                | 15 +++++++++++++--
 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init4.C | 10 ++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init4.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index ee534b5db21..87ed2a3a648 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -10620,6 +10620,7 @@ cp_parser_lambda_introducer (cp_parser* parser, tree 
lambda_expr)
 
   /* Record default capture mode.  "[&" "[=" "[&," "[=,"  */
   if (cp_lexer_next_token_is (parser->lexer, CPP_AND)
+      && !cp_lexer_nth_token_is (parser->lexer, 2, CPP_ELLIPSIS)
       && !cp_lexer_nth_token_is (parser->lexer, 2, CPP_NAME)
       && !cp_lexer_nth_token_is_keyword (parser->lexer, 2, RID_THIS))
     LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda_expr) = CPLD_REFERENCE;
@@ -10715,6 +10716,13 @@ cp_parser_lambda_introducer (cp_parser* parser, tree 
lambda_expr)
          continue;
        }
 
+      /* Remember whether we want to capture as a reference or not.  */
+      if (cp_lexer_next_token_is (parser->lexer, CPP_AND))
+       {
+         capture_kind = BY_REFERENCE;
+         cp_lexer_consume_token (parser->lexer);
+       }
+
       bool init_pack_expansion = false;
       location_t ellipsis_loc = UNKNOWN_LOCATION;
       if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
@@ -10727,9 +10735,12 @@ cp_parser_lambda_introducer (cp_parser* parser, tree 
lambda_expr)
          init_pack_expansion = true;
        }
 
-      /* Remember whether we want to capture as a reference or not.  */
-      if (cp_lexer_next_token_is (parser->lexer, CPP_AND))
+      /* Early C++20 drafts had ...& instead of &...; be forgiving.  */
+      if (init_pack_expansion && capture_kind != BY_REFERENCE
+         && cp_lexer_next_token_is (parser->lexer, CPP_AND))
        {
+         pedwarn (cp_lexer_peek_token (parser->lexer)->location,
+                  0, "%<&%> should come before %<...%>");
          capture_kind = BY_REFERENCE;
          cp_lexer_consume_token (parser->lexer);
        }
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init4.C 
b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init4.C
new file mode 100644
index 00000000000..e7c815b92d4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init4.C
@@ -0,0 +1,10 @@
+// P2095R0
+// { dg-do compile { target c++2a } }
+// { dg-options "" }
+
+template <class... T>
+void f(T... t)
+{
+  [&...x=t]{};
+  [...&x=t]{};                 // { dg-warning "7:&" }
+}

base-commit: f1a681a174cdfb82e62c246d6f4add9a25fc2e43
-- 
2.18.1