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author | Richard Eisenberg <rae@richarde.dev> | 2020-01-21 17:52:48 +0000 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2020-03-20 20:42:56 -0400 |
commit | 73a7383ebc17f495d7acd04007c8c56b46532cb6 (patch) | |
tree | b3c9cabb3dc8ae0e7808fda0d65fa8696ebe1570 /utils/describe-unexpected | |
parent | cb1785d9f839e34a3a4892f354f0c51cc6553c0e (diff) | |
download | haskell-73a7383ebc17f495d7acd04007c8c56b46532cb6.tar.gz |
Simplify treatment of heterogeneous equality
Previously, if we had a [W] (a :: k1) ~ (rhs :: k2), we would
spit out a [D] k1 ~ k2 and part the W as irreducible, hoping for
a unification. But we needn't do this. Instead, we now spit out
a [W] co :: k2 ~ k1 and then use co to cast the rhs of the original
Wanted. This means that we retain the connection between the
spat-out constraint and the original.
The problem with this new approach is that we cannot use the
casted equality for substitution; it's too like wanteds-rewriting-
wanteds. So, we forbid CTyEqCans that mention coercion holes.
All the details are in Note [Equalities with incompatible kinds]
in TcCanonical.
There are a few knock-on effects, documented where they occur.
While debugging an error in this patch, Simon and I ran into
infelicities in how patterns and matches are printed; we made
small improvements.
This patch includes mitigations for #17828, which causes spurious
pattern-match warnings. When #17828 is fixed, these lines should
be removed.
Diffstat (limited to 'utils/describe-unexpected')
0 files changed, 0 insertions, 0 deletions