Address corner cases in next_transition_instant (JuliaTime#192)

No longer fails with `FixedTimeZone`s or instants where no future
transition exists.

src/discovery.jl

@@ -126,11 +126,12 @@ end
 
 
 """
-    next_transition_instant(zdt::ZonedDateTime) -> ZonedDateTime
-    next_transition_instant(tz::TimeZone=localzone()) -> ZonedDateTime
+    next_transition_instant(zdt::ZonedDateTime) -> Union{ZonedDateTime, Nothing}
+    next_transition_instant(tz::TimeZone=localzone()) -> Union{ZonedDateTime, Nothing}
 
 Determine the next instant at which a time zone transition occurs (typically
-due to daylight-savings time).
+due to daylight-savings time). If no there exists no future transition then `nothing` will
+be returned.
 
 Note that the provided `ZonedDateTime` isn't normally constructable:
 
@@ -153,13 +154,16 @@ next_transition_instant
 
 function next_transition_instant(zdt::ZonedDateTime)
     tz = zdt.timezone
+    tz isa VariableTimeZone || return nothing
 
     # Determine the index of the transition which occurs after the UTC datetime specified
     index = searchsortedfirst(
         tz.transitions, TimeZones.utc(zdt),
         by=el -> isa(el, TimeZones.Transition) ? el.utc_datetime : el,
     )
 
+    index <= length(tz.transitions) || return nothing
+
     # Use the UTC datetime of the transition and the offset information prior to the
     # transition to create a `ZonedDateTime` which cannot be constructed with the high-level
     # constructors. The instant constructed is equivalent to the first instant after the
@@ -208,7 +212,18 @@ Transition To:     2011-12-31T00:00:00.000+14:00
 show_next_transition
 
 function show_next_transition(io::IO, zdt::ZonedDateTime)
+    if timezone(zdt) isa FixedTimeZone
+        @warn "No transitions exist in time zone $(timezone(zdt))"
+        return
+    end
+
     instant = next_transition_instant(zdt)
+
+    if instant === nothing
+        @warn "No transition exists in $(timezone(zdt)) after: $zdt"
+        return
+    end
+
     epsilon = eps(instant)
     from, to = instant - epsilon, instant + zero(epsilon)
     direction = value(to.zone.offset - from.zone.offset) < 0 ? "Backward" : "Forward"

test/discovery.jl

@@ -68,6 +68,19 @@ paris = first(compile("Europe/Paris", tzdata["europe"]))
             end
         end
     end
+
+    @testset "fixed time zone" begin
+        @test next_transition_instant(ZonedDateTime(2018, 1, 1, tz"UTC")) === nothing
+    end
+
+    @testset "no future transition" begin
+        # Determine the last transition instant for the time zone
+        t = wpg.transitions[end]
+        last_trans_instant = ZonedDateTime(t.utc_datetime, wpg, t.zone)
+
+        @test next_transition_instant(last_trans_instant) !== nothing
+        @test next_transition_instant(last_trans_instant + Millisecond(1)) === nothing
+    end
 end
 
 @testset "show_next_transition" begin
@@ -123,4 +136,22 @@ end
             end
         end
     end
+
+    @testset "fixed time zone" begin
+        msg = "No transitions exist in time zone UTC"
+        instant = ZonedDateTime(2019 ,1, 1, tz"UTC")
+
+        io = IOBuffer()
+        @test_logs (:warn, msg) show_next_transition(io, instant)
+        @test isempty(read(seekstart(io), String))
+    end
+
+    @testset "no future transition" begin
+        msg = "No transition exists in America/Winnipeg after: 2038-03-14T01:59:59.999-06:00"
+        instant = ZonedDateTime(wpg.cutoff - Millisecond(1), wpg; from_utc=true)
+
+        io = IOBuffer()
+        @test_logs (:warn, msg) show_next_transition(io, instant)
+        @test isempty(read(seekstart(io), String))
+    end
 end