[LLVMbugs] [Bug 22588] New: Error when explicitly specify template parameters for variadic template function that uses variadic arguments in return type specification

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22588

            Bug ID: 22588
           Summary: Error when explicitly specify template parameters for
                    variadic template function that uses variadic
                    arguments in return type specification
           Product: clang
           Version: 3.5
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yyc1992 at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 13864
  --> http://llvm.org/bugs/attachment.cgi?id=13864&action=edit
source file

Might be related to 22191 but at least the symptom is different.

Source file that produces the error (Also attached).

```
#include <utility>
#include <functional>

template<typename Func, typename... Args>
static inline auto
caller(Func &&func, Args&&... args)
    -> decltype(func(std::forward<Args>(args)...))
// Changes to `-> void` suppresses the error
{
    return func(std::forward<Args>(args)...);
}

template<typename Func, typename... Args>
static inline auto
wrapper(Func &&func, Args&&... args)
    -> decltype(caller(std::forward<Func>(func),
                       std::forward<Args>(args)...))
{
    return caller<Func, Args...>(std::forward<Func>(func),
                                 std::forward<Args>(args)...);
}

int
main()
{
    wrapper([] () {}); // OK
    wrapper([] (void*) {}, nullptr); // Error on Clang 3.5.1
    return 0;
}
```

Error message produced on Clang 3.5.1
```
main2.cpp:18:12: error: no matching function for call to 'caller'
    return caller<Func, Args...>(std::forward<Func>(func),
           ^~~~~~~~~~~~~~~~~~~~~
main2.cpp:26:5: note: in instantiation of function template specialization
      'wrapper<(lambda at main2.cpp:26:13), nullptr_t>' requested here
    wrapper([] (void*) {}, nullptr);
    ^
main2.cpp:6:1: note: candidate template ignored: substitution failure [with
Func =
      (lambda at main2.cpp:26:13), Args = <nullptr_t>]: pack expansion contains
      parameter packs 'Args' and 'args' that have different lengths (1 vs. 2)
caller(Func &&func, Args&&... args)
^
1 error generated.
```

GCC 4.9.2 compiles it without an error.

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