Ömer Sinan Ağacan authored May 24, 2019 and Marge Bot committed Jun 01, 2019

Fixes #16449.

5341edf3 removed a code in rewrite rules for bit shifts, which broke the
"silly shift guard", causing generating invalid bit shifts or heap
overflow in compile time while trying to evaluate those invalid bit
shifts.

The "guard" is explained in Note [Guarding against silly shifts] in
PrelRules.hs.

More specifically, this was the breaking change:

    --- a/compiler/prelude/PrelRules.hs
    +++ b/compiler/prelude/PrelRules.hs
    @@ -474,12 +474,11 @@ shiftRule shift_op
            ; case e1 of
                _ | shift_len == 0
                  -> return e1
    -             | shift_len < 0 || wordSizeInBits dflags < shift_len
    -             -> return (mkRuntimeErrorApp rUNTIME_ERROR_ID wordPrimTy
    -                                        ("Bad shift length" ++ show shift_len))

This patch reverts this change.

Two new tests added:

- T16449_1: The original reproducer in #16449. This was previously
  casing a heap overflow in compile time when CmmOpt tries to evaluate
  the large (invalid) bit shift in compile time, using `Integer` as the
  result type. Now it builds as expected. We now generate an error for
  the shift as expected.

- T16449_2: Tests code generator for large (invalid) bit shifts.