[cxx-interop] Ban ObjCBool from being substituted into C++ templates

[egorzhdan]

This fixes a compiler crash when calling a templated C++ function with a parameter of type ObjCBool from Swift. The compiler now emits an error for this.

Previously the following would happen:

1. Swift starts to emit SILGen for a call expression takeTAsConstRef(ObjCBool(true)).
2. takeTAsConstRef is a templated C++ function, so Swift asks Clang to instantiate a takeTAsConstRef with a built-in Obj-C boolean type.
3. Swift gets an instantiated function template back that looks like this: void takeTAsConstRef(_Bool t) { ... }.
4. Swift's ClangImporter begins to import that instantiated function.
5. Since the parameter type is not spelled as BOOL, the parameter is not imported as ObjCBool. Instead, it's imported as regular Swift Bool.
6. Swift realizes that the types don't match (ObjCBool vs Bool), tries to apply any of the known implicit conversions, fails to do so, crashes.

rdar://130424969

[egorzhdan]

@swift-ci please test

[egorzhdan]

@swift-ci please test

[ravikandhadai]

Is there a way to customize this error message for this case? Can we emit a fix-it that says use .boolValue to convert to a Swift boolean?

[egorzhdan]

Emitting a fix-it seems like a bit like an overkill for this case, I think. We would also need to handle possible explicit generic parameters (e.g. myCxxMethod<T1, ObjCBool, T2>).
This bit of logic also doesn't have access to source locations. We could certainly pipe them through, but I'm not sure if it's worth the effort.

[beccadax]

Question. I notice this line in ClangTypeConverter::visitStructType() (ClangTypeConverter.cpp:265 or so):

  CHECK_NAMED_TYPE("ObjCBool", ctx.ObjCBuiltinBoolTy);

If you change ctx.ObjCBuiltinBoolTy to ctx.getBOOLType(), do these templates start importing correctly? I think that might help explain why ObjCBool isn't round-tripping.

If that doesn't help, this PR looks like a good band-aid to prevent the crash, but it seems like a better fix ought to be possible in the future.

[egorzhdan]

If you change ctx.ObjCBuiltinBoolTy to ctx.getBOOLType(), do these templates start importing correctly? I think that might help explain why ObjCBool isn't round-tripping.

That's a good question. I initially tried that, and found out that ctx.getBOOLType() returns nullptr until Clang's Sema processes an Obj-C BOOL literal, and then does a name lookup for BOOL. I tried duplicating that logic in this patch, but unfortunately the name lookup won't find BOOL unless objc.h is transitively included in the C++ module which is being processed.