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

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.)

	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/ChangeLog

@@ -1,3 +1,9 @@
+2020-05-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94255
+	* parser.c (cp_parser_class_specifier_1): Check that the scope is
+	nested inside current scope before pushing it.
+
 2020-05-07  Marek Polacek  <polacek@redhat.com>
 
 	  P1957R2

gcc/cp/parser.c

@@ -23873,7 +23873,12 @@ 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);
 

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-07  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94255
+	* g++.dg/template/spec41.C: New test.
+
 2020-05-07  Marek Polacek  <polacek@redhat.com>
 
 	P1957R2

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" }
+  }
+}