Implicit quantification in type synonyms: enable warning by default, add test for #24090

sheafrequested to merge
sheaf/ghc:T24090 into master

This MR:

  • adds a test for #24090 (closed), which was fixed in 0d4ee209,
  • turns the implicit-rhs-quantification warning flag on by default, and adjusts the error message to make it clearer it will become an error very soon

The next step is #25911, which should be tackled for GHC 9.16 (or perhaps 9.18).

Edited by sheaf

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