[LLVMbugs] [Bug 18389] New: Unnecessary warning: "array subscript is of type 'char' [-Wchar-subscripts]" with constant char indexes

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18389

            Bug ID: 18389
           Summary: Unnecessary warning: "array subscript is of type
                    'char' [-Wchar-subscripts]" with constant char indexes
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: chris+llvm.org at qwirx.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The following code for example generates the warning:

BackupQueries.cpp:2092:29: warning: array subscript is of type 'char'
[-Wchar-subscripts]
        bool MachineReadable = opts['m'];

Because 'm' is in ASCII, and therefore a positive char value, on all platforms
that GCC and LLVM support and almost certainly on all platforms implementing
C++11, this warning seems unnecessary, and clearing it requires obfuscating the
code thus:

        bool MachineReadable = opts[(unsigned char)'m'];

We could probably choose not to show this warning when the value of the index
is known by the compiler to be positive.

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