[Bug c++/51385] New: Unnecessary instantiation converting to pointer to template class instance

[hstong at ca dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

             Bug #: 51385
           Summary: Unnecessary instantiation converting to pointer to
                    template class instance
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: hst...@ca.ibm.com
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu


The test case below uses SFINAE to check whether pointers to certain types
can be implicitly converted.

When a template class which is expensive to instantiate is used as the type
pointed-to by the target of the conversion, GCC takes a long time to compile.

I do not believe that the instantiation of the type in question is necessary to
determine the validity of the conversion (notwithstanding the non-normative
note
in 3.2 [basic.def.odr] paragraph 4). See Core Issue 50 regarding that note.


### Self-contained source:$ cat bb2.C
#ifndef SLOW
enum { Depth = 3 };
#else
enum { Depth = 15 };
#endif

template <unsigned N> struct NTmpl;
template <typename T, typename U, typename V = NTmpl<sizeof(T *)> >
struct PtrConvs {
   enum { bad = 1 };
};

template <typename Target, typename Source>
struct PtrConvs<Target, Source, NTmpl<sizeof (*(Target **)0 = (Source *)0)> >;

template <unsigned N, typename T> struct FastGrowingTemplate;

template <typename T>
struct FastGrowingTemplate<0, T> { };

template <unsigned N, typename T>
struct FastGrowingTemplate :
   FastGrowingTemplate<N - 1, const T *>,
   FastGrowingTemplate<N - 1, volatile T *>
{ };

struct B { };

typedef char chk[1];
typedef char chk[PtrConvs<FastGrowingTemplate<Depth, short>, B>::bad];


### Time without -DSLOW:$ time /data/gcc/bin/g++-4.6.0 bb2.C -c

real    0m0.236s
user    0m0.007s
sys     0m0.006s


### Time with -DSLOW:$ time /data/gcc/bin/g++-4.6.0 bb2.C -c -DSLOW

real    1m45.826s
user    1m42.505s
sys     0m0.312s


### g++ -v output:$ /data/gcc/bin/g++-4.6.0 -v
Using built-in specs.
COLLECT_GCC=/data/gcc/bin/g++-4.6.0
COLLECT_LTO_WRAPPER=/data/gcc/libexec/gcc/powerpc64-unknown-linux-gnu/4.6.0/lto-wrapper
Target: powerpc64-unknown-linux-gnu
Configured with: ./configure --prefix=/data/gcc --program-suffix=-4.6.0
--disable-libssp --disable-libgcj --enable-version-specific-runtime-libs
--with-cpu=default32 --enable-secureplt --with-long-double-128 --enable-shared
--enable-__cxa_atexit --enable-threads=posix --enable-languages=c,c++,fortran
--with-gmp=/usr/local
Thread model: posix
gcc version 4.6.0 (GCC)