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Make the 'IsString (Const a b)' instance polykinded on 'b'

crobbins requested to merge trac-crobbins/ghc:is-string-const-polykinded into master

The IsString instance for Const is quite useful, although it currently does not support other kinds in the b position. This adds polykinded support for IsString (Const a b).

I've left a TODO for the release version in the instance haddock. I haven't currently added a test case but would be happy to do so if necessary.

  • are either individually buildable or squashed
  • have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
  • have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
  • add a testcase to the testsuite.

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