Support comparison between all Period subtypes (#354)

* Support comparison between all Period subtypes

Note that defining `==` methods with `Base.:(==)` does not work on
Julia 0.4

* Add entry to README

README.md

@@ -171,6 +171,8 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `Compat.StringVector` is supported on 0.5 and below. On 0.6 and later, it aliases `Base.StringVector`. This function allocates a `Vector{UInt8}` whose data can be made into a `String` in constant time; that is, without copying. On 0.5 and later, use `String(...)` with the vector allocated by `StringVector` as an argument to create a string without copying. Note that if 0.4 support is needed, `Compat.UTF8String(...)` should be used instead. ([#19449])
 
+* `==(::Period, ::Period)` and `isless(::Period, ::Period)` is supported for 0.5 and below. Earlier versions of Julia only supported limited comparison methods between Periods which did not support comparing custom Period subtypes. ([#21378])
+
 ## Renamed functions
 
 * `pointer_to_array` and `pointer_to_string` have been replaced with `unsafe_wrap(Array, ...)` and `unsafe_wrap(String, ...)` respectively

src/Compat.jl

@@ -1492,6 +1492,20 @@ else
     const collect = Base.collect
 end
 
+# https://github.com/JuliaLang/julia/pull/21378
+if VERSION < v"0.6.0-pre.beta.455"
+    import Base: ==, isless
+
+    ==(x::Dates.Period, y::Dates.Period) = (==)(promote(x, y)...)
+    isless(x::Dates.Period, y::Dates.Period) = isless(promote(x,y)...)
+
+    # disallow comparing fixed to other periods
+    ==(x::Dates.FixedPeriod, y::Dates.OtherPeriod) = throw(MethodError(==, (x, y)))
+    ==(x::Dates.OtherPeriod, y::Dates.FixedPeriod) = throw(MethodError(==, (x, y)))
+    isless(x::Dates.FixedPeriod, y::Dates.OtherPeriod) = throw(MethodError(isless, (x, y)))
+    isless(x::Dates.OtherPeriod, y::Dates.FixedPeriod) = throw(MethodError(isless, (x, y)))
+end
+
 include("to-be-deprecated.jl")
 
 # https://github.com/JuliaLang/julia/pull/21746

test/runtests.jl

@@ -1849,6 +1849,27 @@ let foo() = begin
     @test foo() == 2
 end
 
+# PR 21378
+let
+    # https://en.wikipedia.org/wiki/Swatch_Internet_Time
+    immutable Beat <: Dates.Period
+        value::Int64
+    end
+
+    Dates.value(b::Beat) = b.value
+    Dates.toms(b::Beat) = Dates.value(b) * 86400
+    Dates._units(b::Beat) = " beat" * (abs(Dates.value(b)) == 1 ? "" : "s")
+    Base.promote_rule(::Type{Dates.Day}, ::Type{Beat}) = Dates.Millisecond
+    Base.convert{T<:Dates.Millisecond}(::Type{T}, b::Beat) = T(Dates.toms(b))
+
+    @test Beat(1000) == Dates.Day(1)
+    @test Beat(1) < Dates.Day(1)
+    @test_throws MethodError Dates.Day(30) == Dates.Month(1)
+    @test_throws MethodError Dates.Month(1) == Dates.Day(30)
+    @test_throws MethodError Dates.Day(1) < Dates.Month(1)
+    @test_throws MethodError Dates.Month(1) < Dates.Day(1)
+end
+
 include("to-be-deprecated.jl")
 
 nothing