[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: unassig...@gcc.gnu.org

        ReportedBy: pub...@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)