New handling of overlapping inst in Safe Haskell
We do much better now due to the newish per-instance flags. Rather than mark any module that uses `-XOverlappingInstances`, `-XIncoherentInstances` or the new `OVERLAP*` pragmas as unsafe, we regard them all as safe and defer the check until an overlap occurs. An type-class method call that involves overlapping instances is considered _unsafe_ when: 1) The most specific instance, Ix, is from a module marked `-XSafe` 2) Ix is an orphan instance or a MPTC 3) At least one instance that Ix overlaps, Iy, is: a) from a different module than Ix AND b) Iy is not marked `OVERLAPPABLE` This check is only enforced in modules compiled with `-XSafe` or `-XTrustworthy`. This fixes Safe Haskell to work with the latest overlapping instance pragmas, and also brings consistent behavior. Previously, Safe Inferred modules behaved differently than `-XSafe` modules.
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