ghc: panic! (the 'impossible' happened)

[José Romildo Malaquias]

Hello.

Today I wrote a small program to experiment with the Applicative
class. The program is supposed to use an "applicative reader", similar
to a "monad reader", to evaluate arithmetic expressions.

But when compiling the program with ghc-7.6.1, I get the following message:

  $ ghc --make applicative-eval
  [1 of 1] Compiling Main             ( applicative-eval.hs, applicative-eval.o )
  ghc: panic! (the 'impossible' happened)
    (GHC version 7.6.1 for x86_64-unknown-linux):
    expectJust cpeBody:collect_args

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

If the line

  eval (Let s a b) = \m -> eval b ((s,eval a m):m)

is commented out, the program compiles without problems.

Is this a known issue with the compiler?

The source code is attached.

Romildo
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