[LLVMbugs] [Bug 20890] New: Variadic template function fails to match forward-declared template with default values

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20890

            Bug ID: 20890
           Summary: Variadic template function fails to match
                    forward-declared template with default values
           Product: new-bugs
           Version: unspecified
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: schrodingersbox at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 13014
  --> http://llvm.org/bugs/attachment.cgi?id=13014&action=edit
Source file which fails to compile due to bug

A function with a variadic template template argument fails to match against a
template class whose default template arguments are forward-declared. For
example the following code fails to compile:

template<typename A, typename B> struct foo;
template<typename A, typename B=int> struct foo { };

template <template <typename, typename...> class Container>
void processVariadic(Container<double> /*elements*/) { }

int main() {
    processVariadic(foo<double>{});
}

Compiling with 'clang++ -std=c++11 bug.cpp', the resulting error is:

bug.cpp:8:5: error: no matching function for call to 'processVariadic'
    processVariadic(foo<double>{});
    ^~~~~~~~~~~~~~~
bug.cpp:5:6: note: candidate template ignored: substitution failure [with
Container = foo]:
      too few template arguments for class template 'foo'
void processVariadic(Container<double> /*elements*/) { }
     ^               ~~~~~~~~~
1 error generated.

The compiler seems to look at the first declaration and expect foo to require
two template parameters, ignoring the later default parameter. If the default
parameter is instead put on the forward declaration, it compiles fine.

Both clang++ 3.6.0 (trunk) and Apple LLVM version 5.1 (clang-503.0.40) (based
on LLVM 3.4svn) fail to compile. GCC (g++ 4.9.1) is able to compile properly.

This issue occurs, for example, in the libc++ implementation of std::list.

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