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<h3 style="margin-top: 20px; margin-bottom: 10px;">
Simon Peyton Jones pushed to branch wip/T20666 at <a href="https://gitlab.haskell.org/ghc/ghc" style="color: #1068bf;">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: bold;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a7b64fc7b5c68975280f3f59139d64eacf449fdd" style="color: #1068bf;">a7b64fc7</a></strong>
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<span> by Richard Eisenberg </span> <i> at 2023-01-06T10:10:04+00:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 0.8125rem; color: #303030; position: relative; font-family: "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; word-break: break-all; word-wrap: break-word; background-color: #fafafa; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dbdbdb;'>Refactor the treatment of loopy superclass dicts
This patch completely re-engineers how we deal with loopy superclass
dictionaries in instance declarations. It fixes #20666.
The highlights are
* Recognise that the loopy-superclass business should use precisely
the Paterson conditions. This is much much nicer. See
Note [Recursive superclasses] in GHC.Tc.TyCl.Instance
* With that in mind, define "Paterson-smaller" in
Note [Paterson conditions] in GHC.Tc.Validity, and the new
data type `PatersonSize` in GHC.Tc.Utils.TcType, along with
functions to compute and compare PatsonSizes
* Use the new PatersonSize stuff when solving superclass constraints
See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
* In GHC.Tc.Solver.Monad.lookupInInerts, add a missing call to
prohibitedSuperClassSolve. This was the original cause of #20666.
* Treat (TypeError "stuff") as having PatersonSize zero. See
Note [Paterson size for type family applications] in GHC.Tc.Utils.TcType.
* Some refactoring way in which redundant constraints are reported; we
don't want to complain about the extra, apparently-redundant
constraints that we must add to an instance decl because of the
loopy-superclass thing. I moved some work from GHC.Tc.Errors to
GHC.Tc.Solver.
* Add a new section to the user manual to describe the loopy
superclass issue and what rules it follows.
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30 changed files:
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<a href="#039e8f5676356ef05c90af828aae48aac7296e47" style="color: #1068bf;">
compiler/GHC/Core/TyCo/FVs.hs
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compiler/GHC/Tc/Deriv.hs
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compiler/GHC/Tc/Errors.hs
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compiler/GHC/Tc/Solver/Canonical.hs
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compiler/GHC/Tc/Solver/InertSet.hs
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compiler/GHC/Tc/TyCl/Instance.hs
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compiler/GHC/Tc/Types/Constraint.hs
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compiler/GHC/Tc/Types/Origin.hs
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compiler/GHC/Tc/Utils/Backpack.hs
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compiler/GHC/Tc/Utils/Instantiate.hs
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compiler/GHC/Tc/Utils/TcMType.hs
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compiler/GHC/Tc/Utils/TcType.hs
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compiler/GHC/Tc/Utils/Unify.hs
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compiler/GHC/Tc/Validity.hs
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<a href="#ed75822e0b6142f196a6abeb623c7669d31b4711" style="color: #1068bf;">
docs/users_guide/exts/instances.rst
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testsuite/tests/deriving/should_compile/T14339.hs
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testsuite/tests/deriving/should_fail/T21302.hs
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testsuite/tests/deriving/should_fail/T8165_fail2.stderr
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testsuite/tests/indexed-types/should_fail/T13271.stderr
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