[C++ Patch/RFC] PR 71569 ("[6/7/8 regression] Crash: External definition of template member from template struct")

[Paolo Carlini]

Hi,

in this P2 ICE on invalid we fail to diagnose an ill-formed variable 
template partial specialization and the assert at the beginning of 
determine_specialization triggers. Indeed, we didn't diagnose the 
problem via check_specialization_scope (there are no template <>, thus 
begin_specialization isn't involved). Simply adding a specific check to 
check_explicit_specialization exactly when we recognize a variable 
template partial specialization seems a safe thing to do and passes 
testing. Not sure if the test itself is as simple as possible or is 
better done somewhere else...

Thanks, Paolo.

//////////////////////

Index: cp/pt.c
===================================================================
--- cp/pt.c     (revision 258059)
+++ cp/pt.c     (working copy)
@@ -2873,7 +2873,12 @@ check_explicit_specialization (tree declarator,
              /* Partial specialization of variable template.  */
              SET_DECL_TEMPLATE_SPECIALIZATION (decl);
              specialization = 1;
-             goto ok;
+             if (template_count && DECL_CLASS_SCOPE_P (decl)
+                 && template_class_depth (DECL_CONTEXT (decl)) > 0)
+               error ("variable template partial specialization %qD does "
+                      "not specialize any template arguments", declarator);
+             else
+               goto ok;
            }
          else if (cxx_dialect < cxx14)
            error ("non-type partial specialization %qD "
Index: testsuite/g++.dg/cpp1y/var-templ58.C
===================================================================
--- testsuite/g++.dg/cpp1y/var-templ58.C        (nonexistent)
+++ testsuite/g++.dg/cpp1y/var-templ58.C        (working copy)
@@ -0,0 +1,13 @@
+// PR c++/71569
+// { dg-do compile { target c++14 } }
+
+template <class T>
+class Foo
+{
+    template <class U>
+    static bool Bar;
+};
+
+template<class T>
+template<class U>
+bool Foo<T>::Bar<U>;  // { dg-error "variable template partial" }