Looks good to me for what it does, though I guess ( (a, b) ) = (1, 2) and mixin("(a, b)") = (1, 2) still lower to tuple(a, b).opAssign(tuple(1,2)) and (c ? (a, b) : (b, a)) = (1, 2) still lowers to (c ? tuple(a, b) : tuple(b, a)).opAssign(tuple(1, 2)).

Somehow it would be nice if it either worked or errored out, though I am not sure what's the best way to go about that.

Perhaps I could merge TupleLiteralExp and UnpackExp and handle this pull's UnpackExp lowering inside AssignExp semantic - that could handle the mixin/parens cases and possibly the conditional one too. There could be a flag allowDeclarations which is set when an ExpStatement contains an AssignExp whose left-hand side is a TupleLiteralExp. Merging the expressions would also allow non-declaration unpacking in an AssignExp which is a sub-expression of another expression.

Anyway I'll set this pull as ready for the record.