Fix convert_approx with multiple nonzero terms (#1152)

Author: blegat

src/Utilities/functions.jl

@@ -1975,7 +1975,7 @@ function convert_approx(::Type{MOI.SingleVariable}, func::MOI.ScalarAffineFuncti
     f = canonical(func)
     i = findfirst(t -> isapprox(t.coefficient, one(T), atol=tol), f.terms)
     if abs(f.constant) > tol || i === nothing ||
-        any(j -> j != i && abs(f.terms[i]) > tol, eachindex(f.terms))
+        any(j -> j != i && abs(f.terms[j].coefficient) > tol, eachindex(f.terms))
         throw(InexactError(:convert_approx, MOI.SingleVariable, func))
     end
     return MOI.SingleVariable(f.terms[i].variable_index)

test/Utilities/functions.jl

@@ -308,16 +308,18 @@ end
             @test f.terms == MOI.ScalarAffineTerm.([2, 4], [x, y])
             @test f.constant == 5
         end
-        f = MOI.ScalarAffineFunction(MOI.ScalarAffineTerm.([2, 4], [x, y]),
-                                     5)
+        f = MOI.ScalarAffineFunction(MOI.ScalarAffineTerm.([1.0, 0.5], [x, y]), 0.5)
         @testset "convert" begin
             @test_throws InexactError convert(MOI.SingleVariable, f)
+            @test_throws InexactError MOIU.convert_approx(MOI.SingleVariable, f)
+            @test MOIU.convert_approx(MOI.SingleVariable, f, tol = 0.5) == MOI.SingleVariable(x)
             quad_f = MOI.ScalarQuadraticFunction(f.terms,
-                                                 MOI.ScalarQuadraticTerm{Int}[],
+                                                 MOI.ScalarQuadraticTerm{Float64}[],
                                                  f.constant)
-            @test convert(MOI.ScalarQuadraticFunction{Int}, f) ≈ quad_f
+            @test convert(MOI.ScalarQuadraticFunction{Float64}, f) ≈ quad_f
             g = convert(MOI.ScalarAffineFunction{Float64}, MOI.SingleVariable(x))
             @test convert(MOI.SingleVariable, g) == MOI.SingleVariable(x)
+            @test MOIU.convert_approx(MOI.SingleVariable, g) == MOI.SingleVariable(x)
         end
         @testset "operate with Float64 coefficient type" begin
             f = MOI.ScalarAffineFunction(MOI.ScalarAffineTerm.([1.0, 4.0],