Prevent infinite recursion of unit by adding a method unit(::Type{Any})

src/utils.jl

@@ -139,9 +139,22 @@ julia> unit(1.0) == NoUnits
 true
 ```
 """
+
+"""
+This is shown if a user has not defined units for their quantity type.
+"""
+const MISSING_UNIT_DEFINITION_MESSAGE =
+    """
+    You tried calling unit on an object that has none defined.
+    Not everything has (or should have) units.
+    If your type should have units, define a method of unit on them.
+    """
+
+@inline unit(x::Any) = throw( DomainError( x, MISSING_UNIT_DEFINITION_MESSAGE ) )
+@inline unit(x::Type{Any}) = throw( DomainError( x, MISSING_UNIT_DEFINITION_MESSAGE ) )
 @inline unit(x::Number) = NoUnits
 @inline unit(x::Type{T}) where {T <: Number} = NoUnits
-@inline unit(x::Type{Union{Missing, T}}) where T = unit(T)
+@inline unit(x::Type{Union{Missing, T}}) where T = unit(T) # Type restriction required here for T, or might result in infinite recursion.
 @inline unit(x::Type{Missing}) = missing
 @inline unit(x::Missing) = missing
 

test/dates.jl

@@ -12,14 +12,14 @@
             @test_throws MethodError dimension(T(1))
             @test_throws MethodError Unitful.numtype(T)
             @test_throws MethodError Unitful.numtype(T(1))
-            @test_throws MethodError unit(T)
-            @test_throws MethodError unit(T(1))
+            @test_throws DomainError unit(T)
+            @test_throws DomainError unit(T(1))
         end
 
         for p = (CompoundPeriod, CompoundPeriod(), CompoundPeriod(Day(1)), CompoundPeriod(Day(1), Hour(-1)))
             @test dimension(p) === 𝐓
             @test_throws MethodError Unitful.numtype(p)
-            @test_throws MethodError unit(p)
+            @test_throws DomainError unit(p)
         end
     end