Joe Groff 27d7117f9f SIL: Type lowering for function values with substituted SIL function types. ...

After converting a function type to its corresponding SIL type by our usual rules, extract
the substituted generic signature and common interface type for all concrete function types
that the function could be called as. We don't currently have any use cases for distinguishing
types at finer granularity than generic class-constrained/generic unconstrained/not-generic, so
to reduce the amount of conversion noise in SIL, generate the most general generic signature
possible, by extracting every position with a dependent generic parameter or associated type thereof
into a separate generic parameter in the substituted signature, discarding all constraints except
for `AnyObject` layout constraints. This way, if `foo<T>(x: (T, T) -> ())` calls
`bar<T, U>(x: (T, U) -> ())` and forwards `x` along, the two closures that only vary in genericity
don't need a conversion even with substituted function types, because they'll both have the same
abstract lowering `<A, B> (A, B) -> ()` with different substitutions.

2019-11-05 12:48:11 -08:00

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