Per-instance UndecidableInstances

The LANGUAGE pragma OverlappingInstances is now a per-instance modifier specifying the manner of overlap. Can (or why can't) this also be done for UndecidableInstances? It gives me an icky feeling to put it at the top of a source file when I just want to allow one specific instance to be undecidable.

My apologies if this request already exists. It is very difficult to find bugs about UndecidableInstances in particular as opposed merely to those containing code with this pragma, which appear to be numerous.

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Trac field	Value
Version	8.2.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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