Add test for Seq.iter

Author: forki

src/fsharp/FSharp.Compiler.Unittests/SeqFusion.fs

@@ -39,4 +39,15 @@ type SeqFusionTestsModule() =
         // evaluate it
         Assert.areEqual [15; 15; 3] (Seq.toList result)
 
+        Assert.areEqual  ["hello"; "5"; "world"; "5"; "!"; "1"] (Seq.toList list)
+
+    [<Test>]
+    member this.FusisonOfMapIntoIterKeepsSideEffectOrder() =
+        let list = List<string>()
+        let data = ["hello"; "world"; "!"]
+        let results = List<int>()
+            
+        Seq.iter (fun x -> results.Add x) (Seq.map (fun x -> list.Add(x.ToString()); x * 3) (Seq.map (fun y -> list.Add y; y.Length) data))
+
+        Assert.areEqual [15; 15; 3] (Seq.toList results)
         Assert.areEqual  ["hello"; "5"; "world"; "5"; "!"; "1"] (Seq.toList list)

src/fsharp/Optimizer.fs

@@ -2619,14 +2619,13 @@ and OptimizeApplication cenv env (f0,f0ty,tyargs,args,m) =
     | _ -> 
         match expr' with
         // Rewrite Seq.map f (Seq.map g) xs into Seq.map (fun x -> f(g x)) xs
-        | Expr.App(Expr.Val(valRef,_,_) as outerSeqMap,ttype1,[_;fOutType],
+        | Expr.App(Expr.Val(outerValRef,_,_) as outerSeqMap,ttype1,[_;fOutType],
                     [(Expr.Lambda(_,None,None,_,_,m1,fRetType) as f)
-                     Expr.App(Expr.Val(valRef2,_,_),_,[gInType;_],
+                     Expr.App(Expr.Val(innerValRef,_,_),_,[gInType;_],
                                 [Expr.Lambda(_,None,None,gVals,g,_,gRetType)
                                  rest],_)],m2) when
-            valRefEq cenv.g valRef cenv.g.seq_map_vref &&
-            valRefEq cenv.g valRef2 cenv.g.seq_map_vref 
-            -> 
+            valRefEq cenv.g innerValRef cenv.g.seq_map_vref &&
+            valRefEq cenv.g outerValRef cenv.g.seq_map_vref -> 
             let newApp = Expr.App(f,TType_fun(gRetType, fRetType),[],[g],m2)
 
             let reduced =