Clang crashes when parsing operator== for an enum parameter with a bool enum base and no enumerator list

[alanzhao1]

llvm commit I have checked out where this is broken (not necessarily the cause): 4a40fa8

Repro steps

test.cpp:

enum Foo : bool;

bool bar(Foo a, Foo b) {
    return a == b;
}

1. Build Clang with LLVM_ENABLE_ASSERTIONS:BOOL=ON
2. Run clang++ -c test.cpp -o test

Expected result:
clang should compile the file

Actual result:

clang crashes with the assertion

clang++: /path/to/llvm/repo/llvm-project/clang/lib/AST/Expr.cpp:1836: bool clang::CastExpr::CastConsistency() const: Assertion `!getType()->isBooleanType() && "unheralded conversion to bool"' failed.

The full stacktrace is in the attached stacktrace.txt.

(I had to change the extension of the .cpp file and the .sh file as GitHub doesn't support those file type extensions as attachments)

stacktrace.txt
test-f893c2.cpp.txt
test-f893c2.sh.txt

[alanzhao1]

This bug was first observed in Chromium: https://crbug.com/1344641

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[alanzhao1]

The failing assert is here:

llvm-project/clang/lib/AST/Expr.cpp

Line 1836 in 3580daa

assert(!getType()->isBooleanType() && "unheralded conversion to bool");

assert(!getType()->isBooleanType() && "unheralded conversion to bool");

The definition of isBooleanType() is:

inline bool Type::isBooleanType() const {
  if (const auto *BT = dyn_cast<BuiltinType>(CanonicalType))
    return BT->getKind() == BuiltinType::Bool;
  return false;
}

Using gdb, we find that BT->getKind() is BuiltinType::Bool for the test file.

In contrast, if we add an enum list to the input (which compiles without crashing):

enum Foo : bool {a, b};

bool bar(Foo a, Foo b) {
    return a == b;
}

gdb shows us that BT->getKind() is BuiltinType::Int.