Doug Gregor bbcaf8c669 [Type checker] Introduce value witness constraints. ...

Introduce a new kind of constraint, the "value witness" constraint,
which captures a reference to a witness for a specific protocol
conformance. It otherwise acts like a more restricted form of a "value
member" constraint, where the specific member is known (as a
ValueDecl*) in advance.

The constraint is effectively dependent on the protocol
conformance itself; if that conformance fails, mark the type variables
in the resolved member type as "holes", so that the conformance
failure does not cascade.

Note that the resolved overload for this constraint always refers to
the requirement, rather than the witness, so we will end up recording
witness-method references in the AST rather than concrete references,
and leave it up to the optimizers to perform devirtualization. This is
demonstrated by the SIL changes needed in tests, and is part of the
wider resilience issue with conformances described by
rdar://problem/22708391.

2020-01-02 12:06:23 -08:00

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