Why "mismatched input" with variable shift amounts for << or >>?

[Geremia]

Why is this syntactically correct:

module ror(in word, in shift, out o)
    o ^= (word + shift)

but

module ror(in word, in shift, out o)
    o ^= (word >> shift)

throws a line 2:18 mismatched input 'shift' expecting {'$', '#', '(', INT} error?

[burgholzer]

Essentially because your code is not a valid SyReC expression according to the SyReC EBNF grammar (see Section 2).
Shift statements expect numbers for the shift amount, not (input) signals.