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<h3>
Ben Gamari pushed to branch wip/backports
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/7795b53b6e7358fb6d27139bf666652277261930">7795b53b</a></strong>
<div>
<span>by Ryan Scott</span>
<i>at 2020-05-30T16:17:46+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Create di_scoped_tvs for associated data family instances properly
See `Note [Associated data family instances and di_scoped_tvs]` in
`GHC.Tc.TyCl.Instance`, which explains all of the moving parts.
Fixes #18055.
(cherry picked from commit cd8409c26d4370bf2cdcd76801974e99a9adf7b0)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/2923cfd64b009182fd8ea1f9a22493f21969c2d9">2923cfd6</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-05-30T16:17:46+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">PmCheck: Adjust recursion depth for inhabitation test
In #17977, we ran into the reduction depth limit of the typechecker.
That was only a symptom of a much broader issue: The recursion depth
of the coverage checker for trying to instantiate strict fields in the
`nonVoid` test was far too high (100, the `defaultMaxTcBound`).
As a result, we were performing quite poorly on `T17977`.
Short of a proper termination analysis to prove emptyness of a type,
we just arbitrarily default to a much lower recursion limit of 3.
Fixes #17977.
(cherry picked from commit ed58d4fdcbc7b4fa8fbdf3d638a8d53c444ef4f2)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/2c54f842f0557bac937ae6e5623fec7f79a9aa44">2c54f842</a></strong>
<div>
<span>by Ryan Scott</span>
<i>at 2020-05-30T16:17:46+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Run checkNewDataCon before constraint-solving newtype constructors
Within `checkValidDataCon`, we used to run `checkValidType` on the
argument types of a newtype constructor before running
`checkNewDataCon`, which ensures that the user does not attempt
non-sensical things such as newtypes with multiple arguments or
constraints. This works out in most situations, but this falls over
on a corner case revealed in #17955:
```hs
newtype T = Coercible () T => T ()
```
`checkValidType`, among other things, peforms an ambiguity check on
the context of a data constructor, and that it turn invokes the
constraint solver. It turns out that there is a special case in the
constraint solver for representational equalities (read: `Coercible`
constraints) that causes newtypes to be unwrapped (see
`Note [Unwrap newtypes first]` in `TcCanonical`). This special case
does not know how to cope with an ill formed newtype like `T`, so
it ends up panicking.
The solution is surprisingly simple: just invoke `checkNewDataCon`
before `checkValidType` to ensure that the illicit newtype
constructor context is detected before the constraint solver can
run amok with it.
Fixes #17955.
(cherry picked from commit a0d8e92e9c9b67426aa139d6bc46363d8940f992)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0071044d220daad20dafd59fe1436ffa825fa9a5">0071044d</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-05-30T16:17:46+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Improve skolemisation
This patch avoids skolemiseUnboundMetaTyVar making
up a fresh Name when it doesn't need to.
See Note [Skolemising and identity]
Improves error messsages for partial type signatures.
(cherry picked from commit d7002bccd7d131f8ee9b1ddcd83d62262622294d)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fb58c8ef6952d9dfadb2624abbc74be6707a47a1">fb58c8ef</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-05-30T16:17:47+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Improve pretty-printing for TyConBinders
In particular, show their kinds.
(cherry picked from commit fa37940cd72f82abc460f5c0a5de64dd75cee6ae)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0e49e13e5d0d5cbc49710226bf21a6fe0d923084">0e49e13e</a></strong>
<div>
<span>by Simon Peyton Jones</span>
<i>at 2020-05-30T16:17:47+00:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Fix scoping of TyCon binders in TcTyClsDecls
This patch fixes #17566 by refactoring the way we decide the final
identity of the tyvars in the TyCons of a possibly-recursive nest
of type and class decls, possibly with associated types.
It's all laid out in
Note [Swizzling the tyvars before generaliseTcTyCon]
Main changes:
* We have to generalise each decl (with its associated types)
all at once: TcTyClsDecls.generaliseTyClDecl
* The main new work is done in TcTyClsDecls.swizzleTcTyConBndrs
* The mysterious TcHsSyn.zonkRecTyVarBndrs dies altogether
Other smaller things:
* A little refactoring, moving bindTyClTyVars from tcTyClDecl1
to tcDataDefn, tcSynRhs, etc. Clearer, reduces the number of
parameters
* Reduce the amount of swizzling required.
Specifically, bindExplicitTKBndrs_Q_Tv doesn't need
to clone a new Name for the TyVarTv, and not
cloning means that in the vasly common case,
swizzleTyConBndrs is a no-op
In detail:
Rename newTyVarTyVar --> cloneTyVarTyVar
Add newTyVarTyTyVar that doesn't clone
Use the non-cloning newTyVarTyVar in
bindExplicitTKBndrs_Q_Tv
Rename newFlexiKindedTyVarTyVar
--> cloneFlexiKindedTyVarTyVar
* Define new utility function and use it
HsDecls.familyDeclName ::
FamilyDecl (GhcPass p) -> IdP (GhcPass p)
Updates haddock submodule.
(cherry picked from commit b9605396f1f1560aea94792646b835cadcb49f45)
</pre>
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compiler/GHC/Hs/Decls.hs
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compiler/GHC/HsToCore/PmCheck/Oracle.hs
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compiler/typecheck/TcHsSyn.hs
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compiler/typecheck/TcHsType.hs
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compiler/typecheck/TcInstDcls.hs
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compiler/typecheck/TcMType.hs
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compiler/typecheck/TcTyClsDecls.hs
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compiler/types/TyCon.hs
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