Panic when splicing a malformed InfixE

Summary

GHC panics when splicing InfixE a op b or UInfixE a op b where op is not a variable. Related issue: #4877 (closed).

Steps to reproduce

Compiling the following file results in a panic:

-- panic.hs

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH

main = print $(uInfixE [|1|] [|id id|] [|2|])

$ stack ghc --compiler=ghc-8.6.5 -- -fforce-recomp panic.hs

[1 of 1] Compiling Main             ( panic.hs, panic.o )
Linking panic ...
Undefined symbols for architecture x86_64:
  "_Main_main_closure", referenced from:
      _ZCMain_main_info in panic.o
      _u4lI_srt in panic.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
`gcc' failed in phase `Linker'. (Exit code: 1)

The behavior is slightly different when using GHCi:

> :set -XTemplateHaskell
> import Language.Haskell.TH
> $(uInfixE [|1|] [|id id|] [|2|])
ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-apple-darwin):
	nameModule
  internal it_a4n5
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/basicTypes/Name.hs:240:3 in ghc:Name

Expected behavior

Not panicking.

Environment

• GHC version used: 8.6.5

Optional:

• Operating System: macOS Mojave
• System Architecture: x64