GHC does not infer type of `tagToEnum#` expression

Compiling this program:

{-# LANGUAGE MagicHash #-}
module TTE where

import GHC.Prim

f = case tagToEnum# 1# of
      True  -> "True"
      False -> "False"

gives a compilation error:

[1 of 1] Compiling TTE              ( tte.hs, tte.o )

tte.hs:6:10:
    tagToEnum# must appear applied to one argument
    In the expression: tagToEnum# 1#
    In the expression:
      case tagToEnum# 1# of {
        True -> "True"
        False -> "False" }
    In an equation for ‛f’:
        f = case tagToEnum# 1# of {
              True -> "True"
              False -> "False" }

To make it work one needs to supply type annotation for tagToEnum#: case tagToEnum# 1# :: Bool of. I would expect however that type will be inferred from type of patterns in the branches. Although I can imagine that things might get complicated for an ill-typed program:

f = case tagToEnum# 1# of
      True -> "True"
      3    -> "What now?"

If it is not possible to infer type of scrutinee based on patterns then I would expect to get more informative error message. Current one - tagToEnum# must appear applied to one argument - is very misleading. When I saw it for the first time it took me about 15 minutes to actually figure out what GHC expects me to do.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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