Specialised substring equality for annotated strs

The fact that only a string without annotations is equal to a
non-annotated string (in order to preserve the transitivity of
equality), makes the generic fallback methods for string comparison
insufficient.

As such, ==(::AnnoatedString, ::AbstractString) is implemented in
annotated.jl, but this issue re-appears when dealing with substrings.

The obvious solution is to just implement a specialised method for
substrings. This does seem potentially a bit whack-a-mole, but I'm
worried that cleverer solutions might come with subtle issues of their
own. For now, let's try the simple and obvious solution, and improve it
later if we can work out a nicer way of handling this issue in general.

base/strings/annotated.jl

@@ -176,6 +176,24 @@ cmp(a::AnnotatedString, b::AnnotatedString) = cmp(a.string, b.string)
 ==(a::AnnotatedString, b::AbstractString) = isempty(a.annotations) && a.string == b
 ==(a::AbstractString, b::AnnotatedString) = isempty(b.annotations) && a == b.string
 
+# To prevent substring equality from hitting the generic fallback
+
+function ==(a::SubString{<:AnnotatedString}, b::SubString{<:AnnotatedString})
+    SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) ==
+        SubString(b.string.string, b.offset, b.ncodeunits, Val(:noshift)) &&
+        annotations(a) == annotations(b)
+end
+
+==(a::SubString{<:AnnotatedString}, b::AnnotatedString) =
+    annotations(a) == annotations(b) && SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) == b.string
+
+==(a::SubString{<:AnnotatedString}, b::AbstractString) =
+    isempty(annotations(a)) && SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) == b
+
+==(a::AbstractString, b::SubString{<:AnnotatedString}) = b == a
+
+==(a::AnnotatedString, b::SubString{<:AnnotatedString}) = b == a
+
 """
     annotatedstring(values...)
 

test/strings/annotated.jl

@@ -12,6 +12,8 @@
     @test "a" * str == Base.AnnotatedString("asome string")
     @test str * "a" == Base.AnnotatedString("some stringa")
     @test str * str == Base.AnnotatedString("some stringsome string")
+    @test str[3:4] == SubString("me")
+    @test SubString("me") == str[3:4]
     Base.annotate!(str, 1:4, :thing => 0x01)
     Base.annotate!(str, 6:11, :other => 0x02)
     Base.annotate!(str, 1:11, :all => 0x03)
@@ -21,6 +23,8 @@
     #  └───┰─────┘
     #     :all
     @test str[3:4] == SubString(str, 3, 4)
+    @test str[3:4] != SubString("me")
+    @test SubString("me") != str[3:4]
     @test Base.AnnotatedString(str[3:4]) ==
         Base.AnnotatedString("me", [(1:2, :thing => 0x01), (1:2, :all => 0x03)])
     @test Base.AnnotatedString(str[3:6]) ==