Variadic pack at the beginning of function parameters not seen as a non-deduced context

[llvmbot]

Bugzilla Link	15799
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

The following simple snippet fails to compile:

template<typename T> struct any{ template<typename U> any(U){} };
 
template<typename... Ts, typename T>
T back(any<Ts>..., T);
 
int main(){
    using _ = decltype(back<int>(true, 42));
}

With:

main.cpp:7:24: error: no matching function for call to 'back'
    using _ = decltype(back<int>(true, 42));
                       ^~~~~~~~~
main.cpp:4:3: note: candidate template ignored: couldn't infer template argument 'T'
T back(any<Ts>..., T);

May be related to http://llvm.org/bugs/show_bug.cgi?id=9020

[Endilll]

Accepted since Clang 4: https://godbolt.org/z/ncsxj4Ebj

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [15799](https://llvm.org/bz15799) | | Version | trunk | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor |

Extended Description

The following simple snippet fails to compile:

template<typename T> struct any{ template<typename U> any(U){} };
 
template<typename... Ts, typename T>
T back(any<Ts>..., T);
 
int main(){
    using _ = decltype(back<int>(true, 42));
}

With:

main.cpp:7:24: error: no matching function for call to 'back'
    using _ = decltype(back<int>(true, 42));
                       ^~~~~~~~~
main.cpp:4:3: note: candidate template ignored: couldn't infer template argument 'T'
T back(any<Ts>..., T);

May be related to http://llvm.org/bugs/show_bug.cgi?id=9020