The IsStruct case is meant to handle cases like append(f()) where f's
result parameters are something like ([]int, int, int). However, at
this point in the compiler we've already rewritten append(f()) into
"tmp1, tmp2, tmp3 := f(); append(tmp1, tmp2, tmp3)".
As further evidence, the t.Elem() is not a valid method call for a
struct type anyway, which would trigger the Fatalf call in Type.Elem
if this code was ever hit.