Slava Pestov 6d84c18ba4 Sema: Check 'where' clause requirements on type witnesses ...

In the included test case, conformance checking of Wrapper : B would
pick up typealias Foo as a witness for the associated type B.Foo.

However, this typealias Foo is defined in a constrained extension where
T : A, and the underlying type references the associated type A.Foo
on T.

The resulting substitution is invalid when the conformance Wrapper : B
is used in a context where T does not conform to A.

Instead, we should ignore this typealias entirely, since it appears
in an unusable constrained extension.

Fixes <rdar://problem/60219705>, <https://bugs.swift.org/browse/SR-12327>,
<https://bugs.swift.org/browse/SR-12663>.

2020-08-15 01:43:13 -04:00

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