Add support for - in convert (#2987)

Author: blegat

src/functions.jl

@@ -1086,9 +1086,27 @@ end
 
 _add_to_function(::ScalarAffineFunction, ::Any) = nothing
 
-# This is a very rough-and-ready conversion function that only works for very
-# basic expressions, such as those created by
-# `convert(ScalarNonlinearFunction, f)`.
+function _to_scalar_affine(::Type{ScalarAffineFunction{T}}, x::Real) where {T}
+    return ScalarAffineFunction{T}(ScalarAffineTerm{T}[], T(x))
+end
+
+function _to_scalar_affine(
+    ::Type{ScalarAffineFunction{T}},
+    x::VariableIndex,
+) where {T}
+    return ScalarAffineFunction{T}(x)
+end
+
+function _to_scalar_affine(
+    ::Type{ScalarAffineFunction{T}},
+    f::ScalarNonlinearFunction,
+) where {T}
+    return convert(ScalarAffineFunction{T}, f)
+end
+
+# This conversion function handles expressions created by
+# `convert(ScalarNonlinearFunction, f)` and also expressions using `:+`, `:-`,
+# and `:*` operators with affine structure.
 function Base.convert(
     ::Type{ScalarAffineFunction{T}},
     f::ScalarNonlinearFunction,
@@ -1097,6 +1115,15 @@ function Base.convert(
         term = convert(ScalarAffineTerm{T}, f)
         return ScalarAffineFunction{T}([term], zero(T))
     end
+    if f.head == :- && length(f.args) == 2
+        lhs = _to_scalar_affine(ScalarAffineFunction{T}, f.args[1])
+        rhs = _to_scalar_affine(ScalarAffineFunction{T}, f.args[2])
+        return Utilities.operate(-, T, lhs, rhs)
+    end
+    if f.head == :- && length(f.args) == 1
+        inner = _to_scalar_affine(ScalarAffineFunction{T}, f.args[1])
+        return Utilities.operate(-, T, inner)
+    end
     if f.head != :+
         throw(InexactError(:convert, ScalarAffineFunction{T}, f))
     end

test/General/test_functions.jl

@@ -371,6 +371,17 @@ function test_convert_ScalarNonlinearFunction_ScalarAffineFunction()
             convert(MOI.ScalarAffineFunction{Float64}, f_error),
         )
     end
+    # Test :- support (binary subtraction and unary negation)
+    f_sub = MOI.ScalarNonlinearFunction(:-, Any[x, 1.0])
+    @test convert(MOI.ScalarAffineFunction{Float64}, f_sub) ≈ 1.0 * x - 1.0
+    f_sub2 = MOI.ScalarNonlinearFunction(
+        :-,
+        Any[MOI.ScalarNonlinearFunction(:+, Any[x, y]), 2.0],
+    )
+    @test convert(MOI.ScalarAffineFunction{Float64}, f_sub2) ≈
+          1.0 * x + 1.0 * y - 2.0
+    f_neg = MOI.ScalarNonlinearFunction(:-, Any[x])
+    @test convert(MOI.ScalarAffineFunction{Float64}, f_neg) ≈ -1.0 * x
     return
 end