[Bug c++/54875] New: Forward declare enums cannot be used as a template argument

[Bug c++/54875] New: Forward declare enums cannot be used as a template argument

[public at alisdairm dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

             Bug #: 54875
           Summary: Forward declare enums cannot be used as a template
                    argument
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: public@alisdairm.net


The following simple code snippet shows the problem:

template<typename T>
using AddConst = T const;

enum FwdEnum : int;

int main() {
   AddConst<FwdEnum> *ptr = nullptr;
}


Trying to instantiate the alias template, in this case to declare a pointer but
the problem is more general, produces an internal compiler error.  The problem
is probably older than the gcc 4.8 I am testing with, but it does give a nice
assertion from the compiler:


main.cpp: In function 'int main()':
main.cpp:7:20: internal compiler error: tree check: expected enumeral_type,
have template_type_parm in tsubst_enum, at cp/pt.c:18923
    AddConst<FwdEnum> *ptr = nullptr;
                    ^

main.cpp:7:20: internal compiler error: Abort trap: 6
g++-mp-4.8: internal compiler error: Abort trap: 6 (program cc1plus)
Abort trap: 6


Version info of the compiler I am testing with:
gcc -v -save-temps -std=c++11

Using built-in specs.
Target: i686-apple-darwin11
Configured with: /private/var/tmp/llvmgcc42/llvmgcc42-2336.11~28/src/configure
--disable-checking --enable-werror
--prefix=/Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2
--mandir=/share/man --enable-languages=c,objc,c++,obj-c++
--program-prefix=llvm- --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/
--with-slibdir=/usr/lib --build=i686-apple-darwin11
--enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~28/dst-llvmCore/Developer/usr/local
--program-prefix=i686-apple-darwin11- --host=x86_64-apple-darwin11
--target=i686-apple-darwin11 --with-gxx-include-dir=/usr/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-10-09 14:23:36 UTC ---
I can reproduce this error with gcc 4.8.0 20120930 (experimental), were I get:

"main.cpp|7|internal compiler error: tree check: expected enumeral_type, have
template_type_parm in tsubst_enum, at cp/pt.c:18886|"

[Note the slight difference in the source code line number]

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dodji at gcc dot gnu.org

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-09 14:38:06 UTC ---
As far as I can see, the alias declaration is essential. As such, let's add
Dodji in CC, maybe he can have a look.

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2012-10-26
         AssignedTo|unassigned at gcc dot       |dodji at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[dodji at seketeli dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

--- Comment #3 from dodji at seketeli dot org <dodji at seketeli dot org> 2012-11-16 12:57:37 UTC ---
I candidate patchlet has been submitted for this at
http://gcc.gnu.org/ml/gcc-patches/2012-11/msg01375.html.

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

--- Comment #4 from Dodji Seketeli <dodji at gcc dot gnu.org> 2012-11-16 15:20:11 UTC ---
Author: dodji
Date: Fri Nov 16 15:20:03 2012
New Revision: 193562

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=193562
Log:
PR c++/54875 -  Error with alias template that resolves to an enum

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.

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.  Note that for the alias template
instantiation case above, template_type points to a TEMPLATE_TYPE_PARM
which name is AddConst.

This is what the 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.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/alias-decl-27.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/54875] Forward declare enums cannot be used as a template argument

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54875

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #5 from Dodji Seketeli <dodji at gcc dot gnu.org> 2012-11-16 15:23:41 UTC ---
Fixed in trunk (4.8)