Fallback implementation of sizehint! for AbstractDict (#44687)

Fixes #44686 + additional haslength check + docs that sizehint! returns the collection.

Author: petvana

base/abstractdict.jl

@@ -217,7 +217,9 @@ Dict{Int64, Int64} with 3 entries:
 """
 function merge!(d::AbstractDict, others::AbstractDict...)
     for other in others
-        haslength(other) && sizehint!(d, length(d) + length(other))
+        if haslength(d) && haslength(other)
+            sizehint!(d, length(d) + length(other))
+        end
         for (k,v) in other
             d[k] = v
         end
@@ -522,6 +524,9 @@ function ==(l::AbstractDict, r::AbstractDict)
     return anymissing ? missing : true
 end
 
+# Fallback implementation
+sizehint!(d::AbstractDict, n) = d
+
 const hasha_seed = UInt === UInt64 ? 0x6d35bb51952d5539 : 0x952d5539
 function hash(a::AbstractDict, h::UInt)
     hv = hasha_seed

base/abstractset.jl

@@ -1,7 +1,7 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 eltype(::Type{<:AbstractSet{T}}) where {T} = @isdefined(T) ? T : Any
-sizehint!(s::AbstractSet, n) = nothing
+sizehint!(s::AbstractSet, n) = s
 
 function copy!(dst::AbstractSet, src::AbstractSet)
     dst === src && return dst

base/array.jl

@@ -1244,7 +1244,7 @@ function resize!(a::Vector, nl::Integer)
 end
 
 """
-    sizehint!(s, n)
+    sizehint!(s, n) -> s
 
 Suggest that collection `s` reserve capacity for at least `n` elements. This can improve performance.
 

test/dict.jl

@@ -1199,6 +1199,7 @@ end
         map!(v->v-1, values(testdict))
         @test testdict[:a] == 0
         @test testdict[:b] == 1
+        @test sizehint!(testdict, 1) === testdict
     end
     @testset "Dict" begin
         testdict = Dict(:a=>1, :b=>2)

test/sets.jl

@@ -853,3 +853,14 @@ end
     @test !((1, 2) ⊊ (1, 2, 2))
     @test !((1, 2, 2) ⊋ (1, 2))
 end
+
+@testset "AbstractSet & Fallback" begin
+    mutable struct TestSet{T} <: AbstractSet{T}
+        set::Set{T}
+        function TestSet{T}() where T
+            new{T}(Set{T}())
+        end
+    end
+    set = TestSet{Any}()
+    @test sizehint!(set, 1) === set
+end