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The testsuite output now contains diagnostic codes, so many tests need
to be updated at once.
We decided it was best to keep the diagnostic codes in the testsuite
output, so that contributors don't inadvertently make changes to the
diagnostic codes.
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This is extends bb5afd3c274011c5ea302210b4c290ec1f83209c to cover
warnings emitted during the desugaring phase.
This implements another part of #10752
Reviewed-by: quchen, bgamari
Differential Revision: https://phabricator.haskell.org/D1954
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Issue a separate warning per redundant (or inaccessible) clause.
This way each warning can have more precice location information
(the location of the clause under consideration and not the whole
match).
I thought that this could be too much but actually the number of
such warnings is bound by the number of cases matched against (in
contrast to the non-exhaustive warnings which may be exponentially
more).
Test Plan: validate
Reviewers: simonpj, austin, bgamari
Reviewed By: bgamari
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D1920
GHC Trac Issues: #8710
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This patch adresses several problems concerned with exhaustiveness and
redundancy checking of pattern matching. The list of improvements includes:
* Making the check type-aware (handles GADTs, Type Families, DataKinds, etc.).
This fixes #4139, #3927, #8970 and other related tickets.
* Making the check laziness-aware. Cases that are overlapped but affect
evaluation are issued now with "Patterns have inaccessible right hand side".
Additionally, "Patterns are overlapped" is now replaced by "Patterns are
redundant".
* Improved messages for literals. This addresses tickets #5724, #2204, etc.
* Improved reasoning concerning cases where simple and overloaded
patterns are matched (See #322).
* Substantially improved reasoning for pattern guards. Addresses #3078.
* OverloadedLists extension does not break exhaustiveness checking anymore
(addresses #9951). Note that in general this cannot be handled but if we know
that an argument has type '[a]', we treat it as a list since, the instance of
'IsList' gives the identity for both 'fromList' and 'toList'. If the type is
not clear or is not the list type, then the check cannot do much still. I am
a bit concerned about OverlappingInstances though, since one may override the
'[a]' instance with e.g. an '[Int]' instance that is not the identity.
* Improved reasoning for nested pattern matching (partial solution). Now we
propagate type and (some) term constraints deeper when checking, so we can
detect more inconsistencies. For example, this is needed for #4139.
I am still not satisfied with several things but I would like to address at
least the following before the next release:
Term constraints are too many and not printed for non-exhaustive matches
(with the exception of literals). This sometimes results in two identical (in
appearance) uncovered warnings. Unless we actually show their difference, I
would like to have a single warning.
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