Can't splice TH quote with infix declaration for name in two different namespaces

Spun off from #13799 (closed) (and #13054 (comment 138023)). This code compiles:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}

infix 5 :*:
data a :*: b = a :*: b

But this code does not:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}

$([d| infix 5 :*:
      data a :*: b = a :*: b
    |])

$ /opt/ghc/8.2.1/bin/ghc -ddump-splices Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )
Bug.hs:(4,3)-(6,6): Splicing declarations
    [d| infix 5 :*:_anM, :*:_anL
        
        data a_anN :*:_anL b_anO = a_anN :*:_anM b_anO |]
  ======>
    infix 5 :*:_a3IL
    infix 5 :*:_a3IK
    data (:*:_a3IK) a_a3IM b_a3IN = a_a3IM :*:_a3IL b_a3IN

Bug.hs:4:3: error:
    Multiple fixity declarations for ‘:*:_a3IL’
    also at  Bug.hs:(4,3)-(6,6)
  |
4 | $([d| infix 5 :*:
  |   ^^^^^^^^^^^^^^^...

Inspecting the -ddump-splices output reveals why: when infix 5 :*: is renamed, because :*: refers to two different names in both the value and type namespaces, it actually gets renamed to infix 5 :*:_anM, :*:_anL, where :*:_anM and :*:_anL are the same name with different uniques.

Normally, this isn't a problem, since feeding infix 5 :*:_anM, :*:_anL to the typechecker works fine. However, when it's spliced in via Template Haskell, it gets fed back into the renamer, where it believes that :*:_anM and :*:_anL are duplicate names, causing the error.

Really, this is just a symptom of the fact that infix declarations are insufficiently powerful to encode information for identical names in different namespaces. But until GHC gains this ability, we need to find some workaround for this problem. My hunch is that we'll need to give GHC the power to recognize these sorts of duplicate fixity declarations in Convert and only emit one actual fixity declaration per set of duplicates. (Disclaimer: I haven't worked out all the fiddly details yet.)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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