Assignement operator bug ?

gistfile1.nim

import macros

type 
    Td = tuple
        a:string
        b:int

proc get_data(d: Td) : string {.compileTime.} =
    result = d.a # Works if a literal string is used here. 
    result &= "aa"  # BUG
    #result = result & "aa" # OK

macro m(s:static[Td]) : stmt =
    echo get_data(s)
    echo get_data(s)
    result = newEmptyNode()

const s=("TT", 3)
m(s)
m(s)