[PATCH] Fix PR c++/71546 - lambda capture fails with "was not declared in this scope"

[Håkon Sandsmark <hsandsmark at gmail dot com>]

Hi GCC developers,

I have attached a proposed patch for fixing PR c++/71546 - lambda
capture fails with "was not declared in this scope".

The patch clears the parser scope after each lambda capture in
cp_parser_lambda_introducer in parser.c. This is based on the
following observations:

Comment about cp_parser::scope in parse.h:
    "This value is not cleared automatically after a name is looked
    up, so we must be careful to clear it before starting a new look
    up sequence.  (If it is not cleared, then `X::Y' followed by `Z'
    will look up `Z' in the scope of `X', rather than the current
    scope.)"

C++14 standard draft N4140 § 5.1.2 paragraph 10:
    "The identifier in a simple-capture is looked up using
    the usual rules for unqualified name lookup (3.4.1);
    each such lookup shall find an entity."

I have compared the test results from a pristine build (with test case
from PR added) with a bootstrapped build with my patch applied (using
x86_64-linux). This is the output I got from the compare_tests tool:

    $ gcc/contrib/compare_tests gcc-pristine-build gcc-patched-build
    # Comparing directories
    ## Dir1=gcc-pristine-build: 6 sum files
    ## Dir2=gcc-patched-build: 6 sum files

    # Comparing 6 common sum files
    ## /bin/sh gcc/contrib/compare_tests  /tmp/gxx-sum1.95415
/tmp/gxx-sum2.95415
    Tests that now work, but didn't before:

    g++.dg/cpp1y/pr71546.C  -std=gnu++14 (test for excess errors)

    # No differences found in 6 common sum files

2018-02-27  Håkon Sandsmark  <hsandsmark@gmail.com>

    * parser.c (cp_parser_lambda_introducer): Clear scope after
      each lambda capture.

    * g++.dg/cpp1y/pr71546.C: New test.

diff --git gcc/cp/parser.c gcc/cp/parser.c
index bcee1214c2f..fc11f9126d3 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -10440,6 +10440,12 @@ cp_parser_lambda_introducer (cp_parser* parser, tree lambda_expr)
 		   capture_init_expr,
 		   /*by_reference_p=*/capture_kind == BY_REFERENCE,
 		   explicit_init_p);
+
+      /* If there is any qualification still in effect, clear it
+       * now; we will be starting fresh with the next capture.  */
+      parser->scope = NULL_TREE;
+      parser->qualifying_scope = NULL_TREE;
+      parser->object_scope = NULL_TREE;
     }
 
   cp_parser_require (parser, CPP_CLOSE_SQUARE, RT_CLOSE_SQUARE);
diff --git gcc/testsuite/g++.dg/cpp1y/pr71546.C gcc/testsuite/g++.dg/cpp1y/pr71546.C
new file mode 100644
index 00000000000..861563aacf9
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1y/pr71546.C
@@ -0,0 +1,11 @@
+// PR c++/71546
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+#include <memory>
+
+int main()
+{
+  int x1;
+  [e = std::make_shared <int> (), x1]() {};
+}