[PATCH] c++: Fix crash with template spec in different namespace [PR94255]

[Marek Polacek via Gcc-patches]

This is an ICE on invalid, because we're specializing S::foo in the
wrong namespace.  cp_parser_class_specifier_1 parses S::foo in M
and then it tries to push the nested-name-specifier of foo, which is
S.  By that, we're breaking the assumption of push_inner_scope that
the pushed scope must be a scope nested inside current scope: current
scope is M, but the namespace context of S is N, and N is not nested
in M, so we fell into an infinite loop in push_inner_scope_r.

(cp_parser_class_head called check_specialization_namespace which already
gave a permerror.)

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

        PR c++/94255
        * parser.c (cp_parser_class_specifier_1): Check that the scope is
        nested inside current scope before pushing it.

        * g++.dg/template/spec41.C: New test.
---
 gcc/cp/parser.c                        |  7 ++++++-
 gcc/testsuite/g++.dg/template/spec41.C | 17 +++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/template/spec41.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d2f3f853cb4..d4482ef5a90 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -23858,7 +23858,12 @@ cp_parser_class_specifier_1 (cp_parser* parser)
   if (nested_name_specifier_p)
     {
       scope = CP_DECL_CONTEXT (TYPE_MAIN_DECL (type));
-      old_scope = push_inner_scope (scope);
+      /* SCOPE must be a scope nested inside current scope.  */
+      if (is_nested_namespace (current_namespace,
+                              decl_namespace_context (scope)))
+       old_scope = push_inner_scope (scope);
+      else
+       nested_name_specifier_p = false;
     }
   type = begin_class_definition (type);
 
diff --git a/gcc/testsuite/g++.dg/template/spec41.C 
b/gcc/testsuite/g++.dg/template/spec41.C
new file mode 100644
index 00000000000..249fde74c3a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/spec41.C
@@ -0,0 +1,17 @@
+// PR c++/94255 - crash with template spec in different namespace.
+// { dg-do compile { target c++11 } }
+
+namespace N {
+  class S {
+    template <typename> struct foo;
+  };
+  namespace M {
+    using S = ::N::S;
+  }
+}
+
+namespace N {
+  namespace M {
+    template <> struct S::foo<int> {}; // { dg-error "specialization of" }
+  }
+}

base-commit: b57e1621eb76ba80c949ad098829aa8171a8c4ab
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA