fix: fix unnecessary oneunit call in get_unit

Author: AayushSabharwal

src/systems/unit_check.jl

@@ -77,7 +77,7 @@ get_unit(x::SciMLBase.NullParameters) = unitless
 get_unit(op::typeof(instream), args) = get_unit(args[1])
 
 function get_unit(op, args) # Fallback
-    result = oneunit(op(get_unit.(args)...))
+    result = op(get_unit.(args)...)
     try
         get_unit(result)
     catch

test/units.jl

@@ -240,3 +240,10 @@ sys = complete(sys)
 @test isequal(ModelingToolkit.getdefault(sys.pt.a), sys.v * sys.τ)
 @test ModelingToolkit.getdefault(sys.v) ≈ 2.0
 @test ModelingToolkit.getdefault(sys.τ) ≈ 3.0
+
+@testset "Issue#3017" begin
+    @constants c = 1 [unit = us"mol/nmol"]
+    @variables k(t) [unit = us"mol/nmol"]
+    @test ModelingToolkit.get_unit(c) == ModelingToolkit.get_unit(-c)
+    @test_nowarn NonlinearSystem([k ~ c], [k], []; name=:example)
+end