[chriscoey]

MiniZinc.Model reported support for some constraints that write cannot handle.

VariableIndex in Semicontinuous, Semiinteger and Parameter were ignored when writing the model, so the solve returned a wrong answer:

using JuMP, MiniZinc
model = GenericModel{Int}(() -> MiniZinc.Optimizer{Int}("chuffed"))
@variable(model, x in Semiinteger(2, 3))
@constraint(model, x <= 1)
@objective(model, Max, x)
optimize!(model)
value(x)  # 1, but x must be 0, 2 or 3, so the answer is 0

Similarly, @variable(model, p in Parameter(2)) was written as var int: p;, with no value.

These threw a MethodError when writing the model:

• ScalarAffineFunction, ScalarQuadraticFunction and ScalarNonlinearFunction in Interval, ZeroOne and Integer
• VectorOfVariables in Reified{LessThan}, Reified{GreaterThan} and Reified{EqualTo}

using JuMP, MiniZinc
model = GenericModel{Int}(() -> MiniZinc.Optimizer{Int}("chuffed"))
@variable(model, 0 <= x <= 1, Int)
@variable(model, 0 <= y <= 1, Int)
@constraint(model, 1 <= x + y <= 2)
optimize!(model)
# ERROR: MethodError: no method matching _write_constraint(::IOStream, ::Set{String}, ::Dict{...}, ::MathOptInterface.ScalarAffineFunction{Int64}, ::MathOptInterface.Interval{Int64})

supports_constraint now returns false for all of these, like the existing VectorAffineFunction override, so the bridges can reformulate them into constraints that write handles. Both examples now return the right answer.

For the interval example, the bridge adds a slack variable: constraint 1*x + 1*y + -1*x3 = 0; with x3 in 1..2. Writing constraint x + y in 1..2; directly would avoid that. I went with the bridges to match the existing override, but I can switch to writing Interval directly if you prefer.

Without bridges, the Interval, ZeroOne and Integer cases still throw the MethodError, because MOI.copy_to doesn't check supports_constraint for them.

Please merge #122 first: with only this PR, p * y * y <= 18 with p in Parameter(2) is written as ((1*p*y + 0 * y) - 18) <= 0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (61a1207) to head (07e55a1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   98.38%   98.39%   +0.01%     
==========================================
  Files           5        5              
  Lines         681      687       +6     
==========================================
+ Hits          670      676       +6     
  Misses         11       11              

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