Adding a "does X conform to Y" request

[dan-zheng]

@Slava_Pestov

TypeChecker::conformsToProtocol() is a static method so you can totally call it from SIL as well as Sema. Even non-static methods can be used now because the TypeChecker instance lives forever (but we're trying to gut and eventually remove the TypeChecker singleton altogether).

The issue is TypeChecker.h lives in lib/Sema instead of include/swift/Sema, so it can't be included/called from SIL.

Could we please move TypeChecker.h to include/swift/Sema? That directory is currently mostly empty for some reason, maybe it's intentional for separation of concerns.

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Thanks for the clarification about lookupConformance and conformsToProtocol!

@Slava_Pestov

I'm wondering what kind of issue you encountered with the other two entry points though. lookupConformnace() is the underlying operation used by TypeChecker::conformsToProtocol(), and ModuleDecl::conformsToProtocol() just calls TypeChecker::conformsToProtocol().

I can't remember specific unresolved issues with ModuleDecl::lookupConformance in SIL but I'll make a note if I come across any. (You previously helped with a SubstitutionMap question - thanks again.)

Type-checking @differentiable attributes where clauses (looking up Differentiable protocol conformances using the right APIs and arguments) has been a longstanding source of issues though. I posted a separate question about @differentiable attribute and cc'd you.