issubset: check IteratorSize trait before calling length (#28871)

(cherry picked from commit 24de931)

base/abstractset.jl

@@ -226,14 +226,15 @@ end
 <=(l::AbstractSet, r::AbstractSet) = l ⊆ r
 
 function issubset(l, r)
-
-    rlen = length(r)
-    #This threshold was empirically determined by repeatedly
-    #sampling using these two methods.
-    lenthresh = 70
-
-    if rlen > lenthresh && !isa(r, AbstractSet)
-       return issubset(l, Set(r))
+    if haslength(r)
+        rlen = length(r)
+        #This threshold was empirically determined by repeatedly
+        #sampling using these two methods (see #26198)
+        lenthresh = 70
+
+        if rlen > lenthresh && !isa(r, AbstractSet)
+            return issubset(l, Set(r))
+        end
     end
 
     for elt in l

test/sets.jl

@@ -604,3 +604,16 @@ end
         end
     end
 end
+
+struct OpenInterval{T}
+    lower::T
+    upper::T
+end
+Base.in(x, i::OpenInterval) = i.lower < x < i.upper
+Base.IteratorSize(::Type{<:OpenInterval}) = Base.SizeUnknown()
+
+@testset "Continuous sets" begin
+    i = OpenInterval(2, 4)
+    @test 3 ∈ i
+    @test issubset(3, i)
+end