occasional linking problems with --make?

[Evan Laforge]

Just as an addendum, this just happened again, but I got new errors
this time.  Make gave the usual:

Linking build/test/RunTests ...
Undefined symbols:
  "_UiziTrackC_d28LF", referenced from:
      _s28UI_info in TrackC.o
  "_UiziRulerC_dk8C", referenced from:
      _skev_info in RulerC.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

But then reloading that same module in ghci gave a new one:

> :r
[ 23 of 124] Compiling Ui.UiMsg         ( Ui/UiMsg.hs, interpreted )

Ui/UiMsg.hs:51:31:
    Can't find interface-file declaration for type constructor or
class Key.Modifier
      Probable cause: bug in .hi-boot file, or inconsistent .hi file
      Use -ddump-if-trace to get an idea of which file caused the error
    In the type `[Key.Modifier]'
    In the definition of data constructor `Mouse'
    In the data type declaration for `Data'

The odd thing is that after restarting ghci and loading there was no
problem, so I didn't get a chance to try its -ddump-if-trace
suggestion.  In this case before the reload I had added an instance
declaration to a type in the Key module, though oddly it wasn't the
referenced Modifier type.

Removing the referenced .o files and recompiling was enough to fix the
link error.