[llvm-bugs] [Bug 49337] New: accept-invalid: __builtin_types_compatible_p pollutes the environment of tag type checking

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49337

            Bug ID: 49337
           Summary: accept-invalid: __builtin_types_compatible_p pollutes
                    the environment of tag type checking
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zhan3299 at purdue.edu
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

I am not sure how to accurately describe the problem. In short,

Following code is obviously invalid, as the use of 'X' with tag type does not
match previous declaration (https://godbolt.org/z/qad8vz).

$ cat case1.c
struct X {
    union X {
        int b;
        long c;
    } d;
} a;

int main() { return 0; }


However, when we put this erroneous declaration into a
__builtin_types_compatible_p function, clang accepts it
(https://godbolt.org/z/TrxTzs). Note that it is rejected by GCC
(https://godbolt.org/z/rWde88).

$ cat case2.c
int main() {
    int a = __builtin_types_compatible_p(int, struct X {
        union X {
            int b;
            long c;
        } d;
    });

    return a;
}


Even worse, this invalid __builtin_types_compatible_p may additionally affects
other statements (https://godbolt.org/z/avxvzj), which, at least, gives the
wrong error reports.

$ cat case3.c
int main() {
    int a = __builtin_types_compatible_p(int, struct X {
        union X {
            int b;
            long c;
        } d;
    });

    enum X { A, B } h;
    h = A;

    return a;
}

$ clang case3.c

#1 with x86-64 clang (trunk)
case3.c:10:9: error: use of undeclared identifier 'A'
    h = A;
        ^
1 error generated.

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