[LLVMbugs] [Bug 21348] New: -Wignored-qualifier noisy on common pattern

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21348

            Bug ID: 21348
           Summary: -Wignored-qualifier noisy on common pattern
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: chisophugis at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In libraries that make use of vectors, it is common to parametrize the type
"best type for passing a vector as an argument" (generally either T or T&). I
believe this has to do with ABI and calling convention preferences; why this is
done is not the topic of this bug though. This also can extend to Point classes
which thinly wrap a vector.

For example (using int instead of a vector type):

#if ....
typedef int &IntArg;
#else // (assume this branch isn't taken)
typedef int IntArg;
#endif

int foo(const IntArg I) {
  return I * 3;
}

Currently, clang's -Wignored-qualifiers fires on such code due to the const
applying to the &. This means that everywhere that `const IntArg` is used
(which is frequently), -Wignored-qualifiers fires, which is excessively noisy.

I think it would make sense to fire only in the case where the user has
explicitly written a & with a const applied to it.

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