added test for the branch we were currently imssing

test/varinfo.jl

@@ -770,4 +770,43 @@ DynamicPPL.getspace(::DynamicPPL.Sampler{MySAlg}) = (:s,)
             @test any(!Base.Fix1(DynamicPPL.istrans, varinfo_linked), vns_m)
         end
     end
+
+    @testset "sampling from linked varinfo" begin
+        # `~`
+        @model function demo(n=1)
+            x = Vector(undef, n)
+            for i in eachindex(x)
+                x[i] ~ Exponential()
+            end
+            return x
+        end
+        model1 = demo(1)
+        varinfo1 = DynamicPPL.link!!(VarInfo(model1), model1)
+        # Sampling from `model2` should hit the `istrans(vi) == true` branches
+        # because all the existing variables are linked.
+        model2 = demo(2)
+        varinfo2 = last(DynamicPPL.evaluate!!(model2, deepcopy(varinfo1), SamplingContext()))
+        for vn in [@varname(x[1]), @varname(x[2])]
+            @test DynamicPPL.istrans(varinfo2, vn)
+        end
+
+        # `.~`
+        @model function demo_dot(n=1)
+            x ~ Exponential()
+            if n > 1
+                y = Vector(undef, n - 1)
+                y .~ Exponential()
+            end
+            return x
+        end
+        model1 = demo_dot(1)
+        varinfo1 = DynamicPPL.link!!(DynamicPPL.untyped_varinfo(model1), model1)
+        # Sampling from `model2` should hit the `istrans(vi) == true` branches
+        # because all the existing variables are linked.
+        model2 = demo_dot(2)
+        varinfo2 = last(DynamicPPL.evaluate!!(model2, deepcopy(varinfo1), SamplingContext()))
+        for vn in [@varname(x), @varname(y[1])]
+            @test DynamicPPL.istrans(varinfo2, vn)
+        end
+    end
 end