lookupIfaceGlobal crash with SOURCE import

Try this (in testsuite/tests/stranal/should_compile)

bash$ ghc -c T8743.hs-boot
bash$ ghc -c T8743.hs
ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
	tcIfaceGlobal (local): not found:
    main:T8743.$fxToRowMaybe{v r2J}
    [(r2L, Class ‘main:T8743.ToRow{tc r2L}’),
     (r2M, Data constructor ‘main:T8743.D:ToRow{d r2M}’),
     (r2P, Identifier ‘main:T8743.D:ToRow{v r2P}’),
     (roB, Identifier ‘main:T8743.toRow{v roB}’),
     (rqT, Coercion axiom ‘main:T8743.NTCo:ToRow{tc rqT}’)]

Something clearly wrong here. It think this happens with prettty much any version of GHC. I found this when poking about in #10181.