Underconstrained typed holes are non-performant

With a fresh build of 8.5:

GHCi, version 8.5.20180323: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/leo/.ghci
Prelude> 3 _ 4
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.5.20180323 for x86_64-apple-darwin):
	ASSERT failed!
  2
  1
  t_a4j6[tau:1]
  Maybe a_a4j7[tau:2]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1157:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1213:5 in ghc:Outputable
        assertPprPanic, called at compiler/typecheck/TcMType.hs:720:54 in ghc:TcMType

I believe this fails in earlier versions as well, but I haven't tested it.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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