[AST-Matchers] The nullPointerConstant matcher is problematic

[GKxxQAQ]

Current definition of clang::ast_matchers::nullPointerConstant (in clang/include/clang/ASTMatchers.h) is

AST_MATCHER_FUNCTION(internal::Matcher<Expr>, nullPointerConstant) {
  return anyOf(
      gnuNullExpr(), cxxNullPtrLiteralExpr(),
      integerLiteral(equals(0), hasParent(expr(hasType(pointerType())))));
}

The third argument to anyOf here is weird: As the C standard (C99, C11, C17) says (6.3.2.3 p3)

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.

So, if nullPointerConstant() is supposed to match any null pointer constant based on this definition, why is it restricted with hasParent(expr(hasType(pointerType())))? If it is only supposed to match null pointer constants that are really used as null pointers, it is still problematic:

int main() {
  int *p[1];
  p[0];
}

In this example, the integer literal 0 in p[0] is matched by expr(nullPointerConstant()), because its parent expression p[0] does have a pointer type.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (GKxxQAQ)

Current definition of `clang::ast_matchers::nullPointerConstant` (in `clang/include/clang/ASTMatchers.h`) is ```cpp AST_MATCHER_FUNCTION(internal::Matcher<Expr>, nullPointerConstant) { return anyOf( gnuNullExpr(), cxxNullPtrLiteralExpr(), integerLiteral(equals(0), hasParent(expr(hasType(pointerType()))))); } ``` The third argument to `anyOf` here is problematic, I think. Counterexample: ```cpp int main() { int *p[1]; p[0]; } ``` The integer literal `0` in `p[0]` is matched by `expr(nullPointerConstant())`.

[Sirraide]

In this example, the integer literal 0 in p[0] is matched by expr(nullPointerConstant()), because its parent expression p[0] does have a pointer type.

Yeah, this seems pretty wrong to me at least; I’m more familiar w/ C++ than C, but it would seem very strange to me candidly if the C standard really considered that 0 in p[0] to be a null pointer constant. I’m also not too familiar w/ AST matchers, but we should probably be able to check for an enclosing (possibly implicit) cast expression instead?

[Sirraide]

Godbolt link: https://godbolt.org/z/jaz4jYohe