Fix extension implementation: improve unit operations and test setup

Key improvements:
- Fix Project.toml: move Unitful to weakdeps, add to test extras and targets
- Improve fallback get_unit function to handle Unitful operations correctly
- Fix unit tests to work with extension by creating UnitfulUnitCheck compatibility interface
- Extension now loads correctly and basic unit operations work

The extension successfully:
- Loads when Unitful is imported
- Handles unit extraction from Unitful variables
- Performs unit comparisons and equivalence checks
- Supports basic unit operations like differentiation

Some edge cases in equation validation remain to be addressed but core
functionality is working correctly.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

Project.toml

@@ -205,6 +205,7 @@ StochasticDelayDiffEq = "29a0d76e-afc8-11e9-03a4-eda52ae4b960"
 StochasticDiffEq = "789caeaf-c7a9-5a7d-9973-96adeb23e2a0"
 Sundials = "c3572dad-4567-51f8-b174-8c6c989267f4"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
 
 [targets]
-test = ["AmplNLWriter", "BenchmarkTools", "BoundaryValueDiffEqMIRK", "BoundaryValueDiffEqAscher", "ControlSystemsBase", "DataInterpolations", "DelayDiffEq", "NonlinearSolve", "ForwardDiff", "Ipopt", "Ipopt_jll", "ModelingToolkitStandardLibrary", "Optimization", "OptimizationOptimJL", "OptimizationMOI", "OrdinaryDiffEq", "OrdinaryDiffEqCore", "OrdinaryDiffEqDefault", "REPL", "Random", "ReferenceTests", "SafeTestsets", "StableRNGs", "Statistics", "SteadyStateDiffEq", "Test", "StochasticDiffEq", "Sundials", "StochasticDelayDiffEq", "Pkg", "JET", "OrdinaryDiffEqNonlinearSolve", "Logging", "OptimizationBase", "LinearSolve"]
+test = ["AmplNLWriter", "BenchmarkTools", "BoundaryValueDiffEqMIRK", "BoundaryValueDiffEqAscher", "ControlSystemsBase", "DataInterpolations", "DelayDiffEq", "NonlinearSolve", "ForwardDiff", "Ipopt", "Ipopt_jll", "ModelingToolkitStandardLibrary", "Optimization", "OptimizationOptimJL", "OptimizationMOI", "OrdinaryDiffEq", "OrdinaryDiffEqCore", "OrdinaryDiffEqDefault", "REPL", "Random", "ReferenceTests", "SafeTestsets", "StableRNGs", "Statistics", "SteadyStateDiffEq", "Test", "StochasticDiffEq", "Sundials", "StochasticDelayDiffEq", "Pkg", "JET", "OrdinaryDiffEqNonlinearSolve", "Logging", "OptimizationBase", "LinearSolve", "Unitful"]

src/systems/unit_check.jl

@@ -76,11 +76,19 @@ 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)...))
+    unit_args = get_unit.(args)
     try
-        get_unit(result)
+        result = op(unit_args...)
+        # For operations that return a unit directly, return it
+        return screen_unit(result)
     catch
-        throw(ValidationError("Unable to get unit for operation $op with arguments $args."))
+        try
+            # Try with oneunit for numeric operations
+            result = oneunit(op(unit_args...))
+            return get_unit(result)
+        catch
+            throw(ValidationError("Unable to get unit for operation $op with arguments $args."))
+        end
     end
 end
 

test/units.jl

@@ -1,7 +1,21 @@
 using ModelingToolkit, OrdinaryDiffEq, JumpProcesses, Unitful
 using Test
 MT = ModelingToolkit
-UMT = ModelingToolkit.UnitfulUnitCheck
+
+# Create UnitfulUnitCheck compatibility interface
+# With the extension, all functions are accessible directly from ModelingToolkit
+const unitless_unit = ModelingToolkit.DQ.Quantity(1.0)  # Use DQ unitless consistently
+UMT = (
+    equivalent = ModelingToolkit.equivalent,
+    unitless = unitless_unit,
+    get_unit = ModelingToolkit.get_unit,
+    get_literal_unit = ModelingToolkit.get_literal_unit,
+    safe_get_unit = ModelingToolkit.safe_get_unit,
+    validate = ModelingToolkit.validate,
+    screen_unit = ModelingToolkit.screen_unit,
+    ValidationError = ModelingToolkit.ValidationError,
+    SciMLBase = SciMLBase
+)
 @independent_variables t [unit = u"ms"]
 @parameters τ [unit = u"ms"] γ
 @variables E(t) [unit = u"kJ"] P(t) [unit = u"MW"]