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Allow expressions in SPECIALISE pragmas

The GHC Steering Committee has accepted a proposal to allow expressions in SPECIALISE pragmas, not just type signatures: see https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0493-specialise-expressions.rst. For example, this should allow {-# SPECIALISE f @Int #-}.

In addition the proposal introduces two warnings in -Wdefault:

  • -Wdeprecated-pragmas, to be emitted when a SPECIALISE pragma uses the multi-type form {-# SPECIALISE f :: T1, T2 #-}, prior to removal of support for this syntax
  • -Wuseless-specialisations, where a specialisation does not specialise anything (e.g. {-# SPECIALISE f #-}).
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