reset maxprobe on empty! (#51595)

As pointed out in
#51594 (comment),
this is necessary for the assertion added in
#49447 to be valid.

Fix #51594

base/dict.jl

@@ -184,6 +184,7 @@ end
  resize!(h.keys, newsz)
  resize!(h.vals, newsz)
  h.ndel = 0
+ h.maxprobe = 0
  return h
  end
 
@@ -259,6 +260,7 @@ function empty!(h::Dict{K,V}) where V where K
  resize!(h.vals, sz)
  h.ndel = 0
  h.count = 0
+ h.maxprobe = 0
  h.age += 1
  h.idxfloor = sz
  return h

test/dict.jl

@@ -1502,3 +1502,24 @@ for T in (Int, Float64, String, Symbol)
  @test_broken Core.Compiler.is_terminates(Base.infer_effects(getindex, (Dict{T,Any}, T)))
  end
 end
+
+struct BadHash
+ i::Int
+end
+Base.hash(::BadHash, ::UInt)=UInt(1)
+@testset "maxprobe reset #51595" begin
+ d = Dict(BadHash(i)=>nothing for i in 1:20)
+ empty!(d)
+ sizehint!(d, 0)
+ @test d.maxprobe < length(d.keys)
+ d[BadHash(1)]=nothing
+ @test !(BadHash(2) in keys(d))
+ d = Dict(BadHash(i)=>nothing for i in 1:20)
+ for _ in 1:20
+ pop!(d)
+ end
+ sizehint!(d, 0)
+ @test d.maxprobe < length(d.keys)
+ d[BadHash(1)]=nothing
+ @test !(BadHash(2) in keys(d))
+end