ghc: panic! Binder's type (SingI Symbol <a String>) /= RHS type (String)

Compilation error message:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.3 for i386-unknown-linux):
	cgLookupPanic (probably invalid Core; try -dcore-lint)
    cobox{v a212} [lid]
    static binds for:
    local binds for:
    main:Debug.Trace.LogTree.Test.$p1AllShow{v r1pg} [gid[ClassOp]]
    main:Debug.Trace.LogTree.Test.$p2AllShow{v r1ph} [gid[ClassOp]]
    main:Debug.Trace.LogTree.Test.$p3AllShow{v r1pi} [gid[ClassOp]]
    main:Debug.Trace.LogTree.Test.$p4AllShow{v r1pj} [gid[ClassOp]]
    main:Debug.Trace.LogTree.Test.$p5AllShow{v r1pk} [gid[ClassOp]]
    main:Debug.Trace.LogTree.Test.$p6AllShow{v r1pl} [gid[ClassOp]]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Error in -dcore-lint:

[10 of 10] Compiling Debug.Trace.LogTree.Test ( src/Debug/Trace/LogTree/Test.hs, tmp/Debug/Trace/LogTree/Test.o )
*** Core Lint errors : in result of Desugar (after optimization) ***
{-# LINE 122 "src/Debug/Trace/LogTree/Test.hs #-}: Warning:
    [RHS of $dSingI_a20r :: GHC.TypeLits.SingI GHC.TypeLits.Symbol "h"]
    The type of this binder doesn't match the type of its RHS: $dSingI_a20r
    Binder's type: GHC.TypeLits.SingI GHC.TypeLits.Symbol "h"
    Rhs type: [GHC.Types.Char]

So, it looks like it's related to !TypeLits. The full -dcore-lint output is attached.

To reproduce:

1. git clone https://github.com/ntc2/haskell-call-trace.git
2. cd haskell-call-trace
3. git checkout break-ghc
4. <install dependencies; see below>
5. make

The dependencies from the .cabal file are:

  build-depends: base >=4.6 && <4.7
               , template-haskell >=2.8 && <2.9
               , ghc-prim >=0.3 && <0.4
               , parsec >=3.1 && <3.2
               , tagged >=0.7 && <0.8
               , transformers >=0.3 && <0.4
               , mtl >=2.1 && <2.2

The problematic file is https://github.com/ntc2/haskell-call-trace/blob/break-ghc/src/Debug/Trace/LogTree/Test.hs . I was in the middle of a program-wide change when I ran into this GHC panic; I was using the type errors to guide my fixes. The Test.hs is not type correct, but I don't see how the stuff I changed as anything to do with my use of type-level strings (!TypeLits). The code in the previous commit compiles.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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