[llvm-bugs] [Bug 47478] New: Missing conversion for unscoped enumeration type whose underlying type is not fixed

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https://bugs.llvm.org/show_bug.cgi?id=47478

            Bug ID: 47478
           Summary: Missing conversion for unscoped enumeration type whose
                    underlying type is not fixed
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cuoq at trust-in-soft.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

The bug report refers to the phrase “an unscoped enumeration type whose
underlying type is not fixed can be converted to the first type from the
following list able to hold their entire value range: int, …” in conv.prom in
C++ standards after 2011.

In the code that follows, GCC chooses as the underlying integer type for the
representation of “enum e” the type unsigned int, and so does Clang. Consider
the function f:

enum e { e1, e2 } e;

long long x, y, z;

char *p;

void f(void) {
    e = (enum e) 4294967295;
    x = (long long) e;
    y = e > e1;
    z = &p[e] - p;
}

Compiler Explorer link: https://gcc.godbolt.org/z/7jrMb7

With this choice or representation for “enum e”, the consensus interpretation
for the phrase from conv.prom is that f should set x to 4294967295 and y to 0,
strange as this may seem.

GCC sets z to -1, and this seems correct according to the same phrase. However,
Clang sets z to 4294967295. Is this voluntary, or was the implication of the
phrase from conv.prom overlooked in this case?

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