Trustworthy label.

While they aren't strictly unsafe, it is a similar situation to
Typeable. There are few instances where a programmer will write their
own instance, and having compiler assurance that the Generic
implementation is correct brings a lot of benefits.

I'd forgotten the possiblity that desugaring could generate
dead dictionary bindings; easily fixed by calling occurAnalyseExpr

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

It has reliably increased with commit 1ae5fa45, and has been stable
since then, so it does not seem to be a fluke. I did not investigate why
that commit might have increased this value.

In the code for Trac #8331 we were not getting a complaint, but
we *were* getting a terrible (and virtually useless) RULE, looking
like
   useAbstractMonad (complicated-dictionary-expresion) = $fuseAbstractMonad
where we wanted
   useAbstractMonad d = $fuseAbstractMonad

This commit improves the desugaring algorithm.  More comments
explain; see Note [Drop dictionary bindings on rule LHS]

Summary:
This code needs more comments, but I believe this is safe.  By
definition I can't have broken anything that was working by turning a
panic into a non-panic anyway.

Test Plan: validate

Reviewers: hvr, simonpj, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D105

GHC Trac Issues: #9329

Summary:
There's an knock-on fix in HeapStackCheck.c which is potentially
scary, but I'm pretty confident is OK.  See comment for details.

Test Plan:
I've run all the STM
tests I can find, including libraries/stm/tests/stm049 with +RTS -N8
and some of the constants bumped to make it more of a stress test.

Reviewers: hvr, rwbarton, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D104

GHC Trac Issues: #9379

* Deprecate -XOverlappingInstances

* Update test suite.  Several tests even had entirely unnecessary
  uses of -XOverlappingInstances

* Update user manual with a careful description of the instance
  resolution story

* Fix an outright bug in the handling of duplidate instances in GHCi,
  which are meant to silently overwrite the earlier duplicate. The
  logic was right for family instances but was both more complicated,
  and plain wrong, for class instances.  (If you are interested, the
  bug was that we were eliminating the duplicate from the InstEnv, but
  not from the [ClsInst] held in tcg_insts.)  Test is ghci044a.

C.f. Trac #9242

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

- Better comments about Generalised Newtype Deriving
  See Note [Bindings for Generalised Newtype Deriving]

- Refactor the interface between TcDeriv and TcGenDeriv,
  to reduce the size of the interface of the latter.

Summary:
In this example we ended up with some code that was only reachable via
an info table, because a branch had been optimised away by the native
code generator.  The register allocator then got confused because it
was only considering the first block of the proc to be an entry point,
when actually any of the info tables are entry points.

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D88

Signed-off-by: Austin Seipp <austin@well-typed.com>

for explicitly-bidirectional pattern synonyms