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This commit also consolidates documentation in the user
manual around UndecidableSuperClasses, UndecidableInstances,
and FlexibleContexts.
Close #19186.
Close #19187.
Test case: typecheck/should_compile/T19186,
typecheck/should_fail/T19187{,a}
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Provoked by Trac #11948, this patch adds a new warning to GHC
-Wsimplifiable-class-constraints
It warns if you write a class constraint in a type signature that
can be simplified by an existing instance declaration. Almost always
this means you should simplify it right now; type inference is very
fragile without it, as #11948 shows.
I've put the warning as on-by-default, but I suppose that if there are
howls of protest we can move it out (as happened for -Wredundant-constraints.
It actually found an example of an over-complicated context in CmmNode.
Quite a few tests use these weird contexts to trigger something else,
so I had to suppress the warning in those.
The 'haskeline' library has a few occurrences of the warning (which
I think should be fixed), so I switched it off for that library in
warnings.mk.
The warning itself is done in TcValidity.check_class_pred.
HOWEVER, when type inference fails we get a type error; and the error
suppresses the (informative) warning. So as things stand, the warning
only happens when it doesn't cause a problem. Not sure what to do
about this, but this patch takes us forward, I think.
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The idea was promted by Trac #9939, but it was Christmas, so I did
some recreational programming that went much further.
The idea is to warn when a constraint in a user-supplied context is
redundant. Everything is described in detail in
Note [Tracking redundant constraints]
in TcSimplify.
Main changes:
* The new ic_status field in an implication, of type ImplicStatus.
It replaces ic_insol, and includes information about redundant
constraints.
* New function TcSimplify.setImplicationStatus sets the ic_status.
* TcSigInfo has sig_report_redundant field to say whenther a
redundant constraint should be reported; and similarly
the FunSigCtxt constructor of UserTypeCtxt
* EvBinds has a field eb_is_given, to record whether it is a given
or wanted binding. Some consequential chagnes to creating an evidence
binding (so that we record whether it is given or wanted).
* AbsBinds field abs_ev_binds is now a *list* of TcEvBiinds;
see Note [Typechecking plan for instance declarations] in
TcInstDcls
* Some significant changes to the type checking of instance
declarations; Note [Typechecking plan for instance declarations]
in TcInstDcls.
* I found that TcErrors.relevantBindings was failing to zonk the
origin of the constraint it was looking at, and hence failing to
find some relevant bindings. Easy to fix, and orthogonal to
everything else, but hard to disentangle.
Some minor refactorig:
* TcMType.newSimpleWanteds moves to Inst, renamed as newWanteds
* TcClassDcl and TcInstDcls now have their own code for typechecking
a method body, rather than sharing a single function. The shared
function (ws TcClassDcl.tcInstanceMethodBody) didn't have much code
and the differences were growing confusing.
* Add new function TcRnMonad.pushLevelAndCaptureConstraints, and
use it
* Add new function Bag.catBagMaybes, and use it in TcSimplify
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The reason is the we don't automatically inherit the FDs of
super-classes.
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