[LLVMbugs] [Bug 13136] New: Clang fails to compile non-type template parameter pack deduction in template alias

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13136

             Bug #: 13136
           Summary: Clang fails to compile non-type template parameter
                    pack deduction in template alias
           Product: clang
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jonathan.sauer at gmx.de
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


The following code does not compile with clang r158654:

template <typename T, T... Numbers>
struct NumberTuple { };

template <unsigned int... Numbers>
using MyNumberTuple = NumberTuple<unsigned int, Numbers...>;

template <typename U, unsigned int... Numbers>
void foo(U&&, MyNumberTuple<Numbers...>)                // Does not compile
//void foo(U&&, NumberTuple<unsigned int, Numbers...>)    // Compiles
{
}

// Compiles iff <foo> compiles
template <typename U, unsigned int... Numbers>
void bar(U&&, NumberTuple<unsigned int, Numbers...>)
{
}

template <unsigned int... Numbers>
void baz(MyNumberTuple<Numbers...>)                // Compiles even with alias
{
}

int main()
{
    foo(1, NumberTuple<unsigned int, 0, 1>());
    bar(1, NumberTuple<unsigned int, 0, 1>());
    baz(NumberTuple<unsigned int, 0, 1>());
}



This results in:

$ ~/LLVM/build/Release+Asserts/bin/clang++ -std=c++11 clang.cpp
clang.cpp:26:5: error: no matching function for call to 'foo'
    foo(1, NumberTuple<unsigned int, 0, 1>());
    ^~~
clang.cpp:8:6: note: candidate function [with U = int, Numbers = <>] not
viable: no known conversion
      from 'NumberTuple<unsigned int, 0, 1>' to 'MyNumberTuple<>'
      (aka 'NumberTuple<unsigned int, >') for 2nd argument;
void foo(U&&, MyNumberTuple<Numbers...>)                // Does not compile
     ^
clang.cpp:27:5: error: no matching function for call to 'bar'
    bar(1, NumberTuple<unsigned int, 0, 1>());
    ^~~
clang.cpp:15:6: note: candidate function [with U = int, Numbers = <>] not
viable: no known
      conversion from 'NumberTuple<unsigned int, 0, 1>' to
'NumberTuple<unsigned int>' for 2nd
      argument;
void bar(U&&, NumberTuple<unsigned int, Numbers...>)
     ^
2 errors generated.


It seems that there are two parts to this issue:

1. Clang fails to correctly deduce the non-type template parameters in the call
to <foo> when <foo> uses a template alias instead of the template itself as the
type of the corresponding function argument. This only happens if the function
has an additional template parameter (as <baz> compiles correctly despite using
a template alias).

2. Recovery from that error does not work correctly, resulting in clang failing
template parameter deduction in the call to <bar>. This can be demonstrated by
commenting line 'Does not compile' and uncommenting line 'Compiles'; after this
change, both function calls compile correctly.

-- 
Configure bugmail: http://llvm.org/bugs/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.