[Bug c++/63283] New: constexpr function called by templated function is not treated as constexpr

[roc at ocallahan dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63283

            Bug ID: 63283
           Summary: constexpr function called by templated function is not
                    treated as constexpr
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roc at ocallahan dot org

[roc@eternity tmp]$ cat test.cc
#include <stdio.h>
constexpr int array_length(int (&array)[3]) { return 3; }
int a[] = { 1, 2, 3 };
template <typename T> int f() {
  struct { int e[array_length(a)]; } t;
  return sizeof(t);
}
int main() { printf("%d\n", f<void>()); }

[roc@eternity tmp]$ g++ -std=c++11 -o ~/tmp/test ~/tmp/test.cc && ~/tmp/test
/home/roc/tmp/test.cc: In function ‘int f()’:
/home/roc/tmp/test.cc:5:33: error: array bound is not an integer constant
before ‘]’ token
   struct { int e[array_length(a)]; } t;
                                 ^
[roc@eternity tmp]$ g++ -v
Using built-in specs.
COLLECT_GCC=/usr/bin/g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.3/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-isl=/builddir/build/BUILD/gcc-4.8.3-20140624/obj-x86_64-redhat-linux/isl-install
--with-cloog=/builddir/build/BUILD/gcc-4.8.3-20140624/obj-x86_64-redhat-linux/cloog-install
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.3 20140624 (Red Hat 4.8.3-1) (GCC) 

My friend also sees this in g++ 4.9.1.

clang++ 3.4 compiles the code successfully. Removing "template <typename T>"
from f() (and changing main() to match) makes the problem go away.