Revamp Backpack/hs-boot handling of type class signatures.
Summary: A basket of fixes and improvements: - The permissible things that one can write in a type class definition in an hsig file has been reduced to encompass the following things: - Methods - Default method signatures (but NOT implementation) - MINIMAL pragma It is no longer necessary nor encouraged to specify that a method has a default if it is mentioned in a MINIMAL pragma; the MINIMAL pragma is assumed to provide the base truth as to what methods need to be implemented when writing instances of a type class. - Handling of default method signatures in hsig was previously buggy, as these identifiers were not exported, so we now treat them similarly to DFuns. - Default methods are merged, where methods with defaults override those without. - MINIMAL pragmas are merged by ORing together pragmas. - Matching has been relaxed: a method with a default can be used to fill a signature which did not declare the method as having a default, and a more relaxed MINIMAL pragma can be used (we check if the signature pragma implies the final implementation pragma, on the way fixing a bug with BooleanFormula.implies, see #13073) Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, bgamari, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2925 GHC Trac Issues: #13041
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