Build more implications
The "non-local error" problem in Trac #14185 was due to the interaction of constraints from different function definitions. This patch makes a start towards fixing it. It adds TcUnify.alwaysBuildImplication to unconditionally build an implication in some cases, to keep the constraints from different functions separate. See the new Note [When to build an implication] in TcUnify. But a lot of error messages change, so for now I have set alwaysBuildImplication = False Result: no operational change at all. I'll get back to it!
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