[PATCH] PR c++/54875 - Error when instantiating an alias template that resolves to an enum

[Dodji Seketeli]

Hello,

Consider this short example:

     1  template<typename T>
     2  using AddConst = T const;
     3
     4  enum FwdEnum : int;
     5
     6  int main() {
     7    AddConst<FwdEnum> *ptr = nullptr;
     8  }

At line 7, when we build the type for AddConst<FwdEnum> in
lookup_template_class_1, the resulting type is the enum FwdEnum.  This
confuses lookup_template_class_1 near the if below, wrongly making it
taking the branch and thus calling tsubst_enum while it shouldn't:

      if (TREE_CODE (t) == ENUMERAL_TYPE && !is_dependent_type)
        /* Now that the type has been registered on the instantiations
           list, we set up the enumerators.  Because the enumeration
           constants may involve the enumeration type itself, we make
           sure to register the type first, and then create the
           constants.  That way, doing tsubst_expr for the enumeration
           constants won't result in recursive calls here; we'll find
           the instantiation and exit above.  */
        tsubst_enum (template_type, t, arglist);

Before the alias template feature, the only reason why TREE_CODE (t)
== ENUMERAL_TYPE would be true is when lookup_template_class_1 is
called for an enum that is a member of a class template.  But that
condition can be also true for an alias template instantiation as
well.

So I guess that condition should be changed to TREE_CODE
(template_type) == ENUMERAL_TYPE, to specifically detect the member
enum of a class template case.

This is what the one-liner patchlet below does.

Bootstrapped and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

        * pt.c (lookup_template_class_1): Look at the type of the
        potential member enum of class template to determine if we are
        actually substituting into a member enum of class template.

gcc/testsuite/

        * g++.dg/cpp0x/alias-decl-27.C: New test.
---
 gcc/cp/pt.c                                |  2 +-
 gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 3c9bb56..101b22d 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -7487,7 +7487,7 @@ lookup_template_class_1 (tree d1, tree arglist, tree 
in_decl, tree context,
        = tree_cons (arglist, t,
                     DECL_TEMPLATE_INSTANTIATIONS (templ));
 
-      if (TREE_CODE (t) == ENUMERAL_TYPE && !is_dependent_type)
+      if (TREE_CODE (template_type) == ENUMERAL_TYPE && !is_dependent_type)
        /* Now that the type has been registered on the instantiations
           list, we set up the enumerators.  Because the enumeration
           constants may involve the enumeration type itself, we make
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C 
b/gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C
new file mode 100644
index 0000000..91208ab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C
@@ -0,0 +1,11 @@
+// Origin: PR c++/54875
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+using AddConst = T const;
+
+enum FwdEnum : int;
+
+int main() {
+  AddConst<FwdEnum> *ptr = nullptr;
+}
-- 
1.7.11.7


-- 
                Dodji