Re: [PATCH] c++: base-clause parsing and implicit 'this' [PR100362]

[Jason Merrill via Gcc-patches]

On 5/3/21 12:48 PM, Patrick Palka wrote:

My r11-6815 change to defer access checking when processing a
base-clause removed a pair of pushclass / popclass calls that seemed to
be unnecessary now that we'd also defer access checking while parsing
the base-clause.

But it turns out this makes a difference in the below testcase, where we
have a local class whose base clause implicitly uses the 'this' of the
enclosing class.  Before r11-6815, while parsing the base-clause of the
local class, maybe_resolve_dummy would fail to resolve the dummy 'this'
object because the current scope would be the local class.  Now, since
the current scope is the lambda, maybe_resolve_dummy succeeds and
returns the 'this' for the enclosing class Qux.  Later, during deferred
instantiation of the local class, we get confused trying to resolve the
access of 'a_' through this non-dummy 'this'.

So this patch just reinstates the calls to pushclass / popclass that
were removed in r11-6815.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk?

OK for trunk and 11.2.

gcc/cp/ChangeLog:

        PR c++/100362
        * parser.c (cp_parser_class_head): Reinstate calls to pushclass
        and popclass during parsing of the base-clause that were
        removed in r11-6815.

gcc/testsuite/ChangeLog:

        PR c++/100362
        * g++.dg/cpp1y/lambda-generic-100362.C: New test.
---
  gcc/cp/parser.c                               |  8 +++++-
  .../g++.dg/cpp1y/lambda-generic-100362.C      | 25 +++++++++++++++++++
  2 files changed, 32 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1y/lambda-generic-100362.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index e1b1617da68..28752611b34 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -25683,7 +25683,13 @@ cp_parser_class_head (cp_parser* parser,
       until the entire list has been seen, as per [class.access.general].  */
    push_deferring_access_checks (dk_deferred);
    if (cp_lexer_next_token_is (parser->lexer, CPP_COLON))
-    bases = cp_parser_base_clause (parser);
+    {
+      if (type)
+       pushclass (type);
+      bases = cp_parser_base_clause (parser);
+      if (type)
+       popclass ();
+    }
    else
      bases = NULL_TREE;

diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-100362.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-100362.C

new file mode 100644
index 00000000000..283c2c9477e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-100362.C
@@ -0,0 +1,25 @@
+// PR c++/100362
+// { dg-do compile { target c++14 } }
+
+template <class>
+struct Qux {
+  struct A { } a_;
+  A f();
+
+  void AsyncOp() {
+    [](auto) {
+      struct local : decltype(a_) {};
+      local ptr;
+    }(0);
+
+    [](auto) {
+      struct local : decltype(f()) {};
+      local ptr;
+    }(0);
+  }
+};
+
+void corge() {
+  Qux<int> qux;
+  qux.AsyncOp();
+}