Limit duplicate export warnings for datatypes

containers sometimes exports data constructors/patterns conditionally (depending on GHC version and whether TESTING is defined). Presently, to avoid a duplicate export warning, it's necessary to write a separate export line for each combination of exported constructors, which is horrible:

module Foo (
#ifdef TESTING
#ifdef USE_PATTERN_SYNONYMS
  Foo (Foo, Pat1, Pat2)
#else
  Foo (Foo)
#endif
#elif USE_PATTERN_SYNONYMS
  Foo (Pat1, Pat2)
#else
  Foo
#endif

or to break up the lines with CPP, which is so horrible I can't even bring myself to write it.

I'd much rather be able to write

module Foo (
   Foo
#ifdef TESTING
  ,Foo(Foo)
#endif
#ifdef USE_PATTERN_SYNONYMS
  ,Foo(Pat1, Pat2)
#endif
  )

The trouble here is that GHC warns about duplicate export of the type Foo. I think there's a pretty simple partial solution: only warn about a type export that is *completely* redundant, adding neither type constructor nor pattern. And offer a way to turn off the redundant export warning entirely for types and classes without turning it off for bindings.

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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