libclang `clang_getFieldDeclBitWidth` assumes the bit width expression is evaluable

[chbaker0]

clang_getFieldDeclBitWidth checks if the provided cursor is a bitfield declaration then proceeds to call FieldDecl::getBitWidthValue which in turn calls Expr::EvaluateKnownConstInt on the bit width expression.

EvaluateKnownConstInt requires the expression not be dependent on any template parameters. However, clang_getFieldDeclBitWidth doesn't account for this, and libclang does not expose an obvious way to check if an expression's value depends on a template parameter.

rust-lang/rust-bindgen#2239 is related

[rnk]

It looks like this API can return -1 as a failure value, so this can be fixed with this condition:
if (FD->isBitField() && FD->getBitWidth()->isValueDependent())
return FD->getBitWidthValue(getCursorContext(C));

[chbaker0]

That's probably a good idea. We could have a separate API to check if the expr's value depends on a template parameter, or the clang_getFieldDeclBitWidth could return a separate error code for this condition. Which seems more reasonable?

[rnk]

I went looking through the API to try to find some existing higher level check for "is this AST node at all dependent on template parameters", but I couldn't find anything. I think this API already returns -1 in case of error, so it's better to make it return -1 to make the existing API safe. I think it also makes sense to add a simple boolean predicate that checks if a bitfield's width is non-dependent, so maybe something like clang_Cursor_isNonDependentBitField.