Minizinc: Trivial equality unsatisfiable - constraints

Question

I have the following minizinc model:
include "globals.mzn";
var 0..9: A_1_1;
var 0..9: A_2_1;
var 0..9: A_3_1;
constraint (A_3_1+A_2_1+A_1_1) = A_1_1;
solve satisfy;
The model should have the trivial solution 0=A_1_1=A_2_1=A_3_1. However, Gecode and other solvers report this as unsatisfiable.
What am I overlooking?

Answer 1

It seems to be a bug in MiniZinc when it translates the model to FlatZinc format. The message given is from MiniZinc:
WARNING: model inconsistency detected
test66.mzn:6:
in binary '=' operator expression
The generated FlatZinc file contains just this:
constraint bool_eq(false,true);
solve satisfy;
and that's why the FlatZinc solvers yield UNSATISFIABLE.
Interestingly, the following model works, using a temporary decision variable, T:
var 0..9: A_1_1;
var 0..9: A_2_1;
var 0..9: A_3_1;
var 0..9: T;
constraint
T = A_3_1 + A_2_1 + A_1_1 /\
T = A_1_1
;
solve satisfy;
The model then yield all 10 solutions with A_1_1 is assigned values from 0 to 9, A_2_1 = A_3_1 = 0, and T is assigned to same value as A_1_1.
However, if T is initialized with A_1_1 then UNSAT is thrown again:
var 0..9: T = A_1_1;
Update: One can note that the following constraint works, i.e. 2 * A_1_1 at the right side:
constraint A_3_1 + A_2_1 + A_1_1 = 2 * A_1_1;

Answer 2

Your constraint seems to be attempting to use assignment operator but the operation maynot be valid. You may try a = b+ c but b+c=a may not be allowed. Alternately you can try to use equality == operator to define your constraint which should work. I don't have program installed to validate but hope it gives you bit more insight to the issue. I would not be surprised if there is bug too, I would be keen to know.