- If P is declared to inherit from Q and Q has an associated type named U, then
isValidTypeInContext() returns true for a type 'T.U' where 'T' is a generic
parameter conforming to P, and 'U' is the unresolved DependentMemberType, and
the type 'T.U' will simplify to the term 'T.[P:U]'.
- However, if completion failed while building the rewrite system for P's
requirement signature, then the requirement signature for P won't have a
rule [P].[Q] => [P]. As a result, getTypeForTerm() will fail when given
'T.[P:U]', because the property map entry for 'T' will not contain a
conformance to Q.
Work around this by manually adding protocol inheritance rules when building
a signature from a protocol whose requirement signature failed to compute.
This was triggered by the test case I added in the previous commit to
test/Generics/non_confluent.swift.
