[LLVMbugs] [Bug 18571] New: Noisy warnings about const-attributed functions used in decltype

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18571

            Bug ID: 18571
           Summary: Noisy warnings about const-attributed functions used
                    in decltype
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: akim.demaille at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

It is very handy to use "decltype" to perform a snifae dispatch, as in this
example:

__attribute__((const))
bool bar()
{
  return true;
}

template <typename T>
auto foo(T t) -> decltype(bar(), bool())
{
  return true;
}

template <typename T>
bool foo(...)
{
  return false;
}

int main()
{
  return foo(23) != true;
}

Unfortunately, clang complains that the function I'm sfinaeing on, bar, is
const, and its result is ignored.  The warning in itself makes sense, but not
here.  In addition, it stutters.

akim at erebus /tmp $ clang++-mp-3.4 -std=c++11 sfinae.cc -Wall
sfinae.cc:8:27: warning: ignoring return value of function declared with const
attribute
      [-Wunused-value]
auto foo(T t) -> decltype(bar(), bool())
                          ^~~
sfinae.cc:8:27: warning: ignoring return value of function declared with const
attribute
      [-Wunused-value]
auto foo(T t) -> decltype(bar(), bool())
                          ^~~
2 warnings generated.

Likewise with 3.5.

$ clang++-mp-3.4 --version
clang version 3.4 (tags/RELEASE_34/final)
Target: x86_64-apple-darwin13.0.0
Thread model: posix
$ clang++-mp-3.5 --version
clang version 3.5 (trunk 198565)
Target: x86_64-apple-darwin13.0.0
Thread model: posix


Btw, I don't know if clang has something similar to gcc's
don't_ignore_the_return_value attribute, but there's probably the same issue.

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