The following program fails to compile with GCC 4.3.3 (but compiles
successfully with Comeau C/C++ 4.3.10.1 and MSVC 15.00.30729.01):
template <typename T1, typename T2>
class B {};
template <typename T>
class D : public B<T, T> {};
template <typename T1, typename T2>
void f(B<T1, T2>*) {}
template <template <typename, typename> class U, typename T1, typename T2>
void g(U<T1, T2>*) {}
template <typename T1, typename T2, template <typename, typename> class U>
void h(U<T1, T2>*) {}
int main()
{
B<long, long> bll;
D<long> dl;
f(&bll);
g(&bll);
h(&bll);
f(&dl);
g(&dl); // error: no matching function for call to g(D<long int>*)
h(&dl); // error: no matching function for call to h(D<long int>*)
return 0;
}
According to ISO/IEC 14882-2003 14.8.2.1.3 [temp.deduct.call]:
"If P is a class, and P has the form template-id, then A can be a derived class
of the deduced A."
And from 14.8.2.4.9 [temp.deduct.type]:
"A template type argument T, a template template argument TT or a template
non-type argument i can be
deduced if P and A have one of the following forms: [...] TT<T> [...]"
Therefore, it seems like this code should be valid.
As a partial workaround, changing the first failing line to "g<B>(&dl);"
eliminates that error without excessive readability burden. A similar
workaround for the second failing line, "h<long, long, B>(&dl);", is rather
burdensome in real-world code (e.g. where arguments T1 and T2 are deeply
composed template types).
$ g++ -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.3-5ubuntu4'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4)
--
Summary: Deduction of template template argument via base class
fails
Product: gcc
Version: 4.3.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: trevor at scurrilous dot com
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42329
