# Surprising Core -> STG transformation on master/Clear up what DataConRep is.

After I rebase I suddenly got panic's in a (up to then) working patch.

What seems to happen is that we start with this core:

```
-- RHS size: {terms: 8, types: 19, coercions: 1, joins: 0/0}
GHC.Types.heq_sel [InlPrag=[~]]
:: forall k0 k1 (a :: k0) (b :: k1). (a ~~ b) => a GHC.Prim.~# b
[GblId[ClassOp],
Arity=1,
Caf=NoCafRefs,
Str=<SP(SL)>,
Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
WorkFree=True, Expandable=True,
Guidance=ALWAYS_IF(arity=1,unsat_ok=False,boring_ok=False)
Tmpl= \ (@k0_10)
(@k1_11)
(@(a_12 :: k0_10))
(@(b_13 :: k1_11))
(v_B1 [Occ=Once1!] :: a_12 ~~ b_13) ->
case v_B1 of { GHC.Types.HEq# v_B2 -> CO: v_B2 }},
RULES: Built in rule for GHC.Types.heq_sel: "Class op heq_sel"]
GHC.Types.heq_sel
= \ (@k0_10)
(@k1_11)
(@(a_12 :: k0_10))
(@(b_13 :: k1_11))
(v_B1 :: a_12 ~~ b_13) ->
case v_B1 of v_B1 { GHC.Types.HEq# v_B2 -> CO: v_B2 }
```

And turn it into this STG:

```
GHC.Types.heq_sel [InlPrag=[~]]
:: forall k0 k1 (a :: k0) (b :: k1).
(a GHC.Types.~~ b) =>
a GHC.Prim.~# b
[GblId[ClassOp],
Arity=1,
Caf=NoCafRefs,
Str=<SP(SL)>,
Unf=OtherCon []] =
\r [v_s5u]
case v_s5u of { GHC.Types.HEq# -> GHC.Prim.coercionToken#; };
```

The problem being that the binder for `v_B2`

in the case alternative disappears.
However `GHC.Types.HEq#`

has dataConRepArity of one. So according to that it *should* have an argument.

So either the binder should exist, or the repArity is wrong. I'm not familiar enough with the relevant implementation to easily say which one.

@simonpj This seems like something that might have been a sideeffect of 6c7fff0b. Does it ring a bell?