[LLVMbugs] [Bug 6708] New: clang should print inputs to constant expressions that cause errors

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=6708

           Summary: clang should print inputs to constant expressions that
                    cause errors
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: quality-of-implementation
          Severity: enhancement
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jyasskin at google.com
                CC: chandlerc at gmail.com, llvmbugs at cs.uiuc.edu,
                    dgregor at apple.com


In metaprogramming, we often make assertions about constant expressions. It
would be nice if clang could explain why the constant expression had a bad
value. I don't know how to format the message I'm asking for though.

One example of the problem is below. It would be nice to see how 'expr' was
computed in the first note.

$ cat ~/tmp/test.cc
template <bool>
struct CompileAssert {
};
#define COMPILE_ASSERT(expr, msg) \
  typedef ::CompileAssert<(bool(expr))> \
      msg[bool(expr) ? 1 : -1]

template<typename T>
void foo(const T& t) {
    COMPILE_ASSERT(
        sizeof(T) > 4, T_is_too_small);
}

int main() {
    foo(3);
}
$ Debug/bin/clang++ -c ~/tmp/test.cc
/Users/jyasskin/tmp/test.cc:10:5: error: array size is negative
    COMPILE_ASSERT(
    ^~~~~~~~~~~~~~~
/Users/jyasskin/tmp/test.cc:6:11: note: instantiated from:
      msg[bool(expr) ? 1 : -1]
          ^~~~~~~~~~~~~~~~~~~
/Users/jyasskin/tmp/test.cc:15:5: note: in instantiation of function template
specialization 'foo<int>' requested here
    foo(3);
    ^
2 diagnostics generated.

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