Enforce unit and dimension type safety

README.md

@@ -332,6 +332,7 @@ type DozenWidgets struct{}
 
 func (DozenWidgets) ToBase(v float64) float64   { return v * 12 }
 func (DozenWidgets) FromBase(v float64) float64 { return v / 12 }
+func (DozenWidgets) Dimension() Widget           { return Widget{} }
 
 widgets := quant.New[Widget](2, DozenWidgets{}).To(DozenWidgets{}) // 2
 ```

quantity_test.go

@@ -418,6 +418,7 @@ type testDozenWidgets struct{}
 
 func (testDozenWidgets) ToBase(v float64) float64   { return v * 12 }
 func (testDozenWidgets) FromBase(v float64) float64 { return v / 12 }
+func (testDozenWidgets) Dimension() testWidgets     { return testWidgets{} }
 
 func TestCustomDimensionAndUnit(t *testing.T) {
 	q := quant.New[testWidgets](2, testDozenWidgets{})

unit.go

@@ -14,6 +14,7 @@ type Dimension interface {
 type Unit[D Dimension] interface {
 	ToBase(float64) float64
 	FromBase(float64) float64
+	Dimension() D
 }
 
 // BuiltinUnit is the concrete unit type used by quant's built-in units.
@@ -30,6 +31,12 @@ func (u BuiltinUnit[D]) FromBase(v float64) float64 {
 	return v/u.factor - u.offset
 }
 
+// Dimension identifies the dimension a built-in unit belongs to. It is part
+// of Unit's method set so a unit cannot be passed to a different dimension.
+func (u BuiltinUnit[D]) Dimension() D {
+	return *new(D)
+}
+
 type scaleUnit[D Dimension] = BuiltinUnit[D]
 
 const (