[C++ PATCH] Avoid bogus errors due to -Wredundant-tags (PR c++/93138)

[Jakub Jelinek]

Hi!

For -Wredundant-tags or -Wmismatched-tags, cp_parser_maybe_warn_enum_key
and cp_parser_check_class_key perform an extra name lookup to check if the
enum/struct/class/union keyword is redundant, but as the testcase shows,
if it is not redundant, but there is e.g. a hidden member with the same
name, it results in bogus error about accessing the member, although that
should be diagnosed only if the non-redundant keyword is not present.

So, I think we need to ensure the lookup doesn't perform nor queue any
access checks.  Furthermore, I don't see the point in performing the lookup
for enums if the warning is disabled (by default), for struct/class/union
we already don't perform it if both warnings are disabled.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2020-01-04  Jakub Jelinek  <ja...@redhat.com>

        PR c++/93138
        * parser.c (cp_parser_check_class_key): Disable access checks for the
        simple name lookup.
        (cp_parser_maybe_warn_enum_key): Likewise.  Return early if
        !warn_redundant_tags.

        * g++.dg/warn/Wredundant-tags-2.C: New test.

--- gcc/cp/parser.c.jj  2020-01-01 12:22:34.078474448 +0100
+++ gcc/cp/parser.c     2020-01-03 10:56:53.899072533 +0100
@@ -30663,11 +30663,15 @@ static void
 cp_parser_maybe_warn_enum_key (cp_parser *parser, location_t key_loc,
                               tree type, rid scoped_key)
 {
+  if (!warn_redundant_tags)
+    return;
+
   tree type_decl = TYPE_MAIN_DECL (type);
   tree name = DECL_NAME (type_decl);
-  /* Look up the NAME to see if it unambiguously refers to the TYPE
-     and set KEY_REDUNDANT if so.  */
+  /* Look up the NAME to see if it unambiguously refers to the TYPE.  */
+  push_deferring_access_checks (dk_no_check);
   tree decl = cp_parser_lookup_name_simple (parser, name, input_location);
+  pop_deferring_access_checks ();
 
   /* The enum-key is redundant for uses of the TYPE that are not
      declarations and for which name lookup returns just the type
@@ -30837,7 +30841,9 @@ cp_parser_check_class_key (cp_parser *pa
   tree name = DECL_NAME (type_decl);
   /* Look up the NAME to see if it unambiguously refers to the TYPE
      and set KEY_REDUNDANT if so.  */
+  push_deferring_access_checks (dk_no_check);
   tree decl = cp_parser_lookup_name_simple (parser, name, input_location);
+  pop_deferring_access_checks ();
 
   /* The class-key is redundant for uses of the CLASS_TYPE that are
      neither definitions of it nor declarations, and for which name
--- gcc/testsuite/g++.dg/warn/Wredundant-tags-2.C.jj    2020-01-03 
10:56:10.408730751 +0100
+++ gcc/testsuite/g++.dg/warn/Wredundant-tags-2.C       2020-01-03 
10:56:00.525880326 +0100
@@ -0,0 +1,18 @@
+// PR c++/93138
+// { dg-do compile }
+// { dg-options "-Wredundant-tags" }
+
+struct Foo
+{
+  enum Kind { a };
+private:
+  Kind Kind;
+};
+enum Foo::Kind foo ();         // { dg-bogus "is private within this 
context|redundant" }
+struct Bar
+{
+  struct Kind { int a; };
+private:
+  Kind Kind;
+};
+struct Bar::Kind bar ();       // { dg-bogus "is private within this 
context|redundant" }

        Jakub