Dates: Error on construction/parsing with empty strings

When attempting to construct a `DateTime`, `Date` or `Time` from an
`AbstractString`, throw an `ArgumentError` if the string is empty.
Likewise, error when `parse`ing an empty string as one of these types,
and return `nothing` from `tryparse`.

This behavior differs from previously.  Before, `Date` and `Time` would
return default values of `Date(1)` and `Time(0)`, respectively, while
`DateTime` would error without a `format` argument.  With a `format`
argument, it would return `DateTime(1)`.  However, this appears not to
have been explicitly intended, but rather a consequence of the way
parsing was implemented; no tests for empty string parsing existed.

This addresses #28090 and #43883; see discussion therein.

Summary of changes:
- Check for empty string in `Base.parse(::DateTime)` and throw if so.
- Change documentation to mention this.
- Add a compat notice to the docs contrasting the old behavior.

NEWS.md

@@ -142,6 +142,9 @@ Standard library changes
 
 #### Dates
 
+* Empty strings are no longer incorrectly parsed as valid `DateTime`s, `Date`s or `Time`s and instead throw an
+  `ArgumentError` in constructors and `parse`, while `nothing` is returned by `tryparse` ([#XXX]).
+
 #### Downloads
 
 #### Statistics

stdlib/Dates/docs/src/index.md

@@ -96,8 +96,7 @@ missing parts of dates and times so long as the preceding parts are given. The o
 default values.  For example, `Date("1981-03", dateformat"y-m-d")` returns `1981-03-01`, whilst
 `Date("31/12", dateformat"d/m/y")` gives `0001-12-31`.  (Note that the default year is
 1 AD/CE.)
-Consequently, an empty string will always return `0001-01-01` for `Date`s,
-and `0001-01-01T00:00:00.000` for `DateTime`s.
+An empty string, however, always throws an `ArgumentError`.
 
 Fixed-width slots are specified by repeating the period character the number of times corresponding
 to the width with no delimiter between characters. So `dateformat"yyyymmdd"` would correspond to a date
@@ -153,14 +152,13 @@ an optional third argument of type `DateFormat` specifying the format; for examp
 `parse(Date, "06.23.2013", dateformat"m.d.y")`, or
 `tryparse(DateTime, "1999-12-31T23:59:59")` which uses the default format.
 The notable difference between the functions is that with [`tryparse`](@ref),
-an error is not thrown if the string is in an invalid format;
-instead `nothing` is returned.  Note however that as with the constructors
-above, empty date and time parts assume
-default values and consequently an empty string (`""`) is valid
-for _any_ `DateFormat`, giving for example a `Date` of `0001-01-01`.  Code
-relying on `parse` or `tryparse` for `Date` and `DateTime` parsing should
-therefore also check whether parsed strings are empty before using the
-result.
+an error is not thrown if the string is empty or in an invalid format;
+instead `nothing` is returned.
+
+!!! compat "Julia 1.9"
+    Before Julia 1.9, empty strings could be passed to constructors and `parse`
+    without error, returning as appropriate `DateTime(1)`, `Date(1)` or `Time(0)`.
+    Likewise, `tryparse` did not return `nothing`.
 
 A full suite of parsing and formatting tests and examples is available in [`stdlib/Dates/test/io.jl`](https://github.com/JuliaLang/julia/blob/master/stdlib/Dates/test/io.jl).
 

stdlib/Dates/src/parse.jl

@@ -198,6 +198,7 @@ end
 
 function Base.parse(::Type{DateTime}, s::AbstractString, df::typeof(ISODateTimeFormat))
     i, end_pos = firstindex(s), lastindex(s)
+    i > end_pos && throw(ArgumentError("Cannot parse an empty string as a DateTime"))
 
     local dy
     dm = dd = Int64(1)
@@ -279,6 +280,7 @@ end
 
 function Base.parse(::Type{T}, str::AbstractString, df::DateFormat=default_format(T)) where T<:TimeType
     pos, len = firstindex(str), lastindex(str)
+    pos > len && throw(ArgumentError("Cannot parse an empty string as a Date or Time"))
     val = tryparsenext_internal(T, str, pos, len, df, true)
     @assert val !== nothing
     values, endpos = val
@@ -287,6 +289,7 @@ end
 
 function Base.tryparse(::Type{T}, str::AbstractString, df::DateFormat=default_format(T)) where T<:TimeType
     pos, len = firstindex(str), lastindex(str)
+    pos > len && return nothing
     res = tryparsenext_internal(T, str, pos, len, df, false)
     res === nothing && return nothing
     values, endpos = res

stdlib/Dates/test/io.jl

@@ -586,4 +586,34 @@ end
     @test (@inferred Nothing g()) == datetime
 end
 
+@testset "Issue #43883: parsing empty strings" begin
+    for (T, name, fmt) in zip(
+            (DateTime, Date, Time),
+            ("DateTime", "Date or Time", "Date or Time"),
+            ("yyyy-mm-ddHHMMSS.s", "yyymmdd", "HHMMSS")
+        )
+        @test_throws ArgumentError T("")
+        @test_throws ArgumentError T("", fmt)
+        @test_throws ArgumentError T("", DateFormat(fmt))
+        try
+            T("")
+            @test false
+        catch err
+            @test err.msg == "Cannot parse an empty string as a $name"
+        end
+
+        @test_throws ArgumentError parse(T, "")
+        @test_throws ArgumentError parse(T, "", DateFormat(fmt))
+        try
+            parse(T, "")
+            @test false
+        catch err
+            @test err.msg == "Cannot parse an empty string as a $name"
+        end
+
+        @test tryparse(T, "") === nothing
+        @test tryparse(T, "", DateFormat(fmt)) === nothing
+    end
+end
+
 end