[PATCH] only handle true class-keys in -Wredundant-tags (PR 93801)

[Martin Sebor]

The -Wredundant-tags warning, new in GCC 10, issues false positives
for some declarations involving typename.  Typename isn't a C++
class-key so the warning is not intended in these cases.  Attached
is a simple fix tested on x86_64-linux.

Martin

PR c++/93801 - False -Wmismatched-tags upon redundant typename

gcc/cp/ChangeLog:

	PR c++/93801
	* parser.c (cp_parser_check_class_key): Only handle true C++ class-keys.

gcc/testsuite/ChangeLog:

	PR c++/93801
	* g++.dg/warn/Wredundant-tags-3.C: New test.

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index e8a536ae22f..03946b6d7b1 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -30995,6 +30995,13 @@ cp_parser_check_class_key (cp_parser *parser, location_t key_loc,
   tree decl = cp_parser_lookup_name_simple (parser, name, input_location);
   pop_deferring_access_checks ();
 
+  /* Only consider the true class-keys below and ignore typename_type,
+     etc. that are not C++ class-keys.  */
+  if (class_key != class_type
+      && class_key != record_type
+      && class_key != union_type)
+    return;
+
   /* The class-key is redundant for uses of the CLASS_TYPE that are
      neither definitions of it nor declarations, and for which name
      lookup returns just the type itself.  */
diff --git a/gcc/testsuite/g++.dg/warn/Wredundant-tags-3.C b/gcc/testsuite/g++.dg/warn/Wredundant-tags-3.C
new file mode 100644
index 00000000000..7b30e949d0c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wredundant-tags-3.C
@@ -0,0 +1,45 @@
+/* PR c++/93801 - False -Wmismatched-tags upon redundant typename
+   { dg-do compile }
+   { dg-options "-Wall -Wredundant-tags" } */
+
+namespace N
+{
+  class C { };
+  enum E { };
+  struct S { };
+  union U { };
+
+  template <int> class TC { };
+  template <int> struct TS { };
+  template <int> union TU { };
+}
+
+N::C c1;
+typename N::C c2;     // { dg-bogus "-Wredundant-tags" }
+class N::C c3;        // { dg-warning "-Wredundant-tags" }
+
+N::E e1;
+typename N::E e2;     // { dg-bogus "-Wredundant-tags" }
+enum N::E e3;         // { dg-warning "-Wredundant-tags" }
+
+N::S s1;
+typename N::S s2;     // { dg-bogus "-Wredundant-tags" }
+struct N::S s3;       // { dg-warning "-Wredundant-tags" }
+
+N::U u1;
+typename N::U u2;     // { dg-bogus "-Wredundant-tags" }
+                      // { dg-bogus "'class' tag used in naming 'union N::U" "pr93809" { xfail *-*-*} .-1 }
+union N::U u3;        // { dg-warning "-Wredundant-tags" }
+
+
+typedef N::TC<0> TC0;
+typedef typename N::TC<0> TC0;
+typedef class N::TC<0> TC0;   // { dg-warning "-Wredundant-tags" "pr93809" { xfail *-*-*} .-1 }
+
+typedef N::TS<0> TS0;
+typedef typename N::TS<0> TS0;
+typedef struct N::TS<0> TS0;  // { dg-warning "-Wredundant-tags" "pr93809" { xfail *-*-*} .-1 }
+
+typedef N::TS<0> TS0;
+typedef typename N::TS<0> TS0;
+typedef struct N::TS<0> TS0;  // { dg-warning "-Wredundant-tags" "pr93809" { xfail *-*-*} .-1 }