[LLVMbugs] [Bug 10314] New: "bitset::operator[](size_t) const" does not return bool

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=10314

           Summary: "bitset::operator[](size_t) const" does not return
                    bool
           Product: libc++
           Version: unspecified
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
        AssignedTo: hhinnant at apple.com
        ReportedBy: jonathan.sauer at gmx.de
                CC: llvmbugs at cs.uiuc.edu


The following program does not compile with clang r134322 and libc++ r134357:

#include <bitset>

template <typename>
struct Foo;

template <> struct Foo<bool> {
    static const bool value = true;
};

template <typename B>
static void bar(B)
{
    Foo<B>::value;
}

int main(int, char**)
{
    std::bitset<32> const    bits;

    bar(bits[0]);
}


This results in:

$ clang -std=c++0x -stdlib=libc++ clang.cpp 
clang.cpp:13:2: error: implicit instantiation of undefined template
      'Foo<std::__1::__bit_const_reference<std::__1::__bitset<1, 32> > >'
        Foo<B>::value;
        ^
clang.cpp:20:2: note: in instantiation of function template specialization
      'bar<std::__1::__bit_const_reference<std::__1::__bitset<1, 32> > >'
requested here
        bar(bits[0]);
        ^
clang.cpp:4:8: note: template is declared here
struct Foo;
       ^
1 error generated.


According to 20.5p1, the signature of "operator[](std::size_t) const" is:

   constexpr bool operator[](size_t pos) const;

libc++'s bitset's current implementation's signature is:

  const_reference operator[](size_t) const,

where "const_reference" is equal to "__bit_const_reference<...>". While it is
implicitely convertible to bool, it is not a bool itself and thus creates
problems when used as a template parameter as in the code above.

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