Assertion failure on FD3 test in TcErrors

Test case typecheck/should_compile/FD3 leads to an ASSERTion error:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.11.20150605 for x86_64-apple-darwin):
	ASSERT failed!
  file compiler/typecheck/TcErrors.hs line 321
  [[D] _ :: a_amP[sk] ~ (String, a_amP[sk]) (CNonCanonical)]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

FD3.log (attached) shows the error happening.

(When originally posting this ticket, I was very confused. The first several comments, through ticket:10533#comment:101025, are now bogus. Please ignore.)

Edited Mar 10, 2019 by Richard Eisenberg

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Assertion failure on FD3 test in TcErrors