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We were falling into an infinite loop when doing the ambiguity
check on a class method, even though we had previously detected
a superclass cycle.  There was code to deal with this, but it
wasn't right.

This just simplifies the error message in cases where there are no useful
equalities in the context

that binds a variable mentioned in the wanted

There is really no point in reporting ones further out; they can't be useful

With the new constraint solver, we don't guarantee to fully-normalise
all constraints (if doing so is not necessary to solve them). So we
may end up with an inferred type like
      f :: [F Int] -> Bool
which could be simplifed to
      f :: [Char] -> Bool
if there is a suitable family instance declaration.  This patch
does this normalisation, in TcBinds.mkExport

Previously we could get constraints in which the untouchables-level did not
strictly increase, which is one of the main invariants!

This patch also simplifies and modularises the tricky case of generalising
an inferred let-binding

Previously the univerally-quantified variables of the DFun were also (bizarrely)
used as the lexically-scoped variables of the instance declaration.  So, for example,
the DFun's type could not be alpha-renamed.  This was an odd restriction, which has
bitten me several times.

This patch does the Right Thing, by adding an ib_tyvars field to the
InstBindings record, which captures the lexically scoped variables.
Easy, robust, nice.  (I think this record probably didn't exist originally,
hence the hack.)

This makes newClsInst (was mkInstance) look more like newFamInst, and simplifies
the plumbing of the overlap flag, and ensures that freshening (required by
the InstEnv stuff) happens in one place.

On the way I also tided up the rather ragged family of tcInstSkolTyVars and
friends.  The result at least has more uniform naming.

This change is just for naming uniformity with the existing downgradeRole

This function previously would expand *data* families even when it was asked
for a *Nominal* coercion.  This patch fixes it, and adds comments.

This is a straight refactoring that puts the generation of unfolding
info in one place, which is a lot tidier

The simplified function is tcSuperClasses;
no need for an implication constraint here

I forget all the details, but I spent some time trying to
understand the current setup, and tried to simplify it a bit

In particular, print a bit of debug info in debug-style and dump-style
Otherwise distinct type variables look the same

This was broken in d94de872 when `join`
was inserted between `Monad`'s Haddock string and the `class Monad m`
definition thereby breaking the association.

This removes the redundant "Minimal complete definition"-block included
in the Haddock comment since Haddock renders the `MINIMAL`-pragma as
well (which has is moved to the start of `class` definition for better
readability of the source code)

Morever, the references to `testBitDefault`, `bitDefault`, and
`popCountDefault` have been moved to the respective methods' Haddock
strings for which they can be used.

[skip ci]

Complete #9759. Use `coerce` to get nicer definitions of `Sum` and
`Product`; update documentation for `First` and `Last`.

Reviewed By: hvr

Differential Revision: https://phabricator.haskell.org/D422

Fixes #9742. Previously, `foldr1` as applied to a list-like
structure would be strict in the spine, and `foldl1` would
be strict in the spine of a snoc-list.

See also

 https://www.haskell.org/pipermail/libraries/2014-October/024035.html

Differential Revision: https://phabricator.haskell.org/D423

thanks to Jan for nudging.

This increases the chance of good code after fusing a left fold. See
ticket #7994 and the new Note [Left folds via right fold]

Differential Revision: https://phabricator.haskell.org/D393

to allow the programer to explictitly set the oneShot flag. This helps
with #7994 and will be used in left folds. Also see
https://ghc.haskell.org/trac/ghc/wiki/OneShot

This commit touches libraries/base/GHC/Event/Manager.hs (which used to
have a local definition of the name oneShot) to avoid a shadowing error.

Differential Revision: https://phabricator.haskell.org/D392

Differential Revision: https://phabricator.haskell.org/D391

since they are disallowed both in class and instance declarations