Fix fixed-length negative year parsing (JuliaLang#54535)

Follow up to JuliaLang#53981. Fixes an issue introduced with negative years and
fixed-with date formats:

```julia
julia> Dates.DateTime("-20240521", "yyyymmdd")
ERROR: ArgumentError: Month: 40 out of range (1:12)
Stacktrace:
 [1] DateTime(y::Int64, m::Int64, d::Int64, h::Int64, mi::Int64, s::Int64, ms::Int64, ampm::Dates.AMPM)
   @ Dates ~/Development/Julia/aarch64/latest/usr/share/julia/stdlib/v1.12/Dates/src/types.jl:246
 [2] parse(::Type{DateTime}, str::String, df::DateFormat{:yyyymmdd, Tuple{Dates.DatePart{'y'}, Dates.DatePart{'m'}, Dates.DatePart{'d'}}})
   @ Dates ~/Development/Julia/aarch64/latest/usr/share/julia/stdlib/v1.12/Dates/src/parse.jl:294
 [3] DateTime(dt::String, format::String; locale::Dates.DateLocale)
   @ Dates ~/Development/Julia/aarch64/latest/usr/share/julia/stdlib/v1.12/Dates/src/io.jl:555
 [4] DateTime(dt::String, format::String)
   @ Dates ~/Development/Julia/aarch64/latest/usr/share/julia/stdlib/v1.12/Dates/src/io.jl:554
 [5] top-level scope
   @ REPL[4]:1
```

This PR makes it so that fixed-width formats require the specified
number of digits. I also decided to only add the sign parsing for years
to running into performance issues with parsing sign information where
it isn't expected.

stdlib/Dates/src/io.jl

@@ -111,7 +111,25 @@ end
 
 ### Parse tokens
 
-for c in "yYmdHIMS"
+for c in "yY"
+    @eval begin
+        @inline function tryparsenext(d::DatePart{$c}, str, i, len)
+            val = tryparsenext_sign(str, i, len)
+            if val !== nothing
+                coefficient, i = val
+            else
+                coefficient = 1
+            end
+            # The sign character does not affect fixed length `DatePart`s
+            val = tryparsenext_base10(str, i, len, min_width(d), max_width(d))
+            val === nothing && return nothing
+            y, ii = val
+            return y * coefficient, ii
+        end
+    end
+end
+
+for c in "mdHIMS"
     @eval begin
         @inline function tryparsenext(d::DatePart{$c}, str, i, len)
             return tryparsenext_base10(str, i, len, min_width(d), max_width(d))

stdlib/Dates/src/parse.jl

@@ -156,18 +156,23 @@ If successful, returns a 2-element tuple `(values, pos)`:
     end
 end
 
+@inline function tryparsenext_sign(str::AbstractString, i::Int, len::Int)
+    i > len && return nothing
+    c, ii = iterate(str, i)::Tuple{Char, Int}
+    if c == '+'
+        return 1, ii
+    elseif c == '-'
+        return -1, ii
+    else
+        return nothing
+    end
+end
+
 @inline function tryparsenext_base10(str::AbstractString, i::Int, len::Int, min_width::Int=1, max_width::Int=0)
     i > len && return nothing
     min_pos = min_width <= 0 ? i : i + min_width - 1
     max_pos = max_width <= 0 ? len : min(i + max_width - 1, len)
     d::Int64 = 0
-    c, neg = iterate(str, i)::Tuple{Char, Int}
-    if c == '-'
-        i = neg
-        neg = -1
-    else
-        neg = 1
-    end
     @inbounds while i <= max_pos
         c, ii = iterate(str, i)::Tuple{Char, Int}
         if '0' <= c <= '9'
@@ -180,7 +185,7 @@ end
     if i <= min_pos
         return nothing
     else
-        return d * neg, i
+        return d, i
     end
 end
 
@@ -207,10 +212,18 @@ function Base.parse(::Type{DateTime}, s::AbstractString, df::typeof(ISODateTimeF
     i, end_pos = firstindex(s), lastindex(s)
     i > end_pos && throw(ArgumentError("Cannot parse an empty string as a DateTime"))
 
+    coefficient = 1
     local dy
     dm = dd = Int64(1)
     th = tm = ts = tms = Int64(0)
 
+    # Optional sign
+    let val = tryparsenext_sign(s, i, end_pos)
+        if val !== nothing
+            coefficient, i = val
+        end
+    end
+
     let val = tryparsenext_base10(s, i, end_pos, 1)
         val === nothing && @goto error
         dy, i = val
@@ -279,7 +292,7 @@ function Base.parse(::Type{DateTime}, s::AbstractString, df::typeof(ISODateTimeF
     end
 
     @label done
-    return DateTime(dy, dm, dd, th, tm, ts, tms)
+    return DateTime(dy * coefficient, dm, dd, th, tm, ts, tms)
 
     @label error
     throw(ArgumentError("Invalid DateTime string"))

stdlib/Dates/test/io.jl

@@ -325,6 +325,23 @@ end
     # From Matt Bauman
     f = "yyyy-mm-ddTHH:MM:SS"
     @test Dates.DateTime("2014-05-28T16:46:04", f) == Dates.DateTime(2014, 5, 28, 16, 46, 04)
+
+    f = "yyyymmdd"
+    @test Dates.DateTime("20240521", f) == Dates.DateTime(2024, 5, 21)
+    @test Dates.DateTime("-20240521", f) == Dates.DateTime(-2024, 5, 21)
+    @test Dates.DateTime("+20240521", f) == Dates.DateTime(2024, 5, 21)
+    f = "YYYYmmdd"
+    @test Dates.DateTime("20240521", f) == Dates.DateTime(2024, 5, 21)
+    @test Dates.DateTime("-20240521", f) == Dates.DateTime(-2024, 5, 21)
+    @test Dates.DateTime("+20240521", f) == Dates.DateTime(2024, 5, 21)
+    f = "-yyyymmdd"
+    @test Dates.DateTime("-20240521", f) == Dates.DateTime(2024, 5, 21)
+    @test_throws ArgumentError Dates.DateTime("+20240521", f)
+    @test_throws ArgumentError Dates.DateTime("20240521", f)
+    f = "-YYYYmmdd"
+    @test Dates.DateTime("-20240521", f) == Dates.DateTime(2024, 5, 21)
+    @test_throws ArgumentError Dates.DateTime("+20240521", f)
+    @test_throws ArgumentError Dates.DateTime("20240521", f)
 end
 
 @testset "Error handling" begin
@@ -403,6 +420,17 @@ end
     @test_throws ArgumentError parse(Date, "Foo, 12 Nov 2016 07:45:36", Dates.RFC1123Format)
 end
 
+@testset "ISODateTimeFormat" begin
+    dt = Dates.DateTime(2024, 5, 21, 10, 57, 22)
+    neg_dt = Dates.DateTime(-2024, 5, 21, 10, 57, 22)
+    @test parse(Dates.DateTime, "2024-05-21T10:57:22", Dates.ISODateTimeFormat) == dt
+    @test parse(Dates.DateTime, "+2024-05-21T10:57:22", Dates.ISODateTimeFormat) == dt
+    @test parse(Dates.DateTime, "-2024-05-21T10:57:22", Dates.ISODateTimeFormat) == neg_dt
+
+    @test_throws ArgumentError parse(Dates.DateTime, "-", Dates.ISODateTimeFormat)
+    @test_throws ArgumentError parse(Dates.DateTime, "+", Dates.ISODateTimeFormat)
+end
+
 @testset "Issue 15195" begin
     f = "YY"
     @test Dates.format(Dates.Date(1999), f) == "1999"