CR feedback

src/Compilers/CSharp/Portable/FlowAnalysis/DataFlowPass.cs

@@ -1577,8 +1577,6 @@ public override BoundNode VisitForEachStatement(BoundForEachStatement node)
             // NOTE: iteration variables are not declared or assigned
             //       before the collection expression is evaluated 
             var result = base.VisitForEachStatement(node);
-
-            // NOTE: do not report unused iteration variables. They are always considered used.
             return result;
         }
 
@@ -2188,6 +2186,7 @@ public override void VisitForEachIterationVariables(BoundForEachStatement node)
                 Debug.Assert((object)iterationVariable != null);
                 int slot = GetOrCreateSlot(iterationVariable);
                 if (slot > 0) SetSlotAssigned(slot);
+                // NOTE: do not report unused iteration variables. They are always considered used.
                 NoteWrite(iterationVariable, null, read: true);
             }
         }

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenDeconstructTests.cs

@@ -6192,5 +6192,56 @@ public static void Test5(IEnumerable<((int, int), string)> en)
             CompileAndVerify(compilation, expectedOutput: "0");
         }
 
+        [Fact]
+        [WorkItem(16106, "https://github.com/dotnet/roslyn/issues/16106")]
+        public void DefAssignmentsStruct002()
+        {
+            string source = @"
+public class MyClass
+{
+    public static void Main()
+    {
+        var data = new int[10];
+        var arr  = new int[2];
+
+        foreach (arr[out int size] in data) {}
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib(source, references: s_valueTupleRefs);
+            compilation.VerifyDiagnostics(
+                // (9,36): error CS0230: Type and identifier are both required in a foreach statement
+                //         foreach (arr[out int size] in data) {}
+                Diagnostic(ErrorCode.ERR_BadForeachDecl, "in").WithLocation(9, 36)
+                );
+        }
+
+        [Fact]
+        [WorkItem(16106, "https://github.com/dotnet/roslyn/issues/16106")]
+        public void DefAssignmentsStruct003()
+        {
+            string source = @"
+public class MyClass
+{
+    public static void Main()
+    {
+        var data = new (int, int)[10];
+        var arr  = new int[2];
+
+        foreach ((arr[out int size], int b) in data) {}
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib(source, references: s_valueTupleRefs);
+            compilation.VerifyDiagnostics(
+                // (9,27): error CS1615: Argument 1 may not be passed with the 'out' keyword
+                //         foreach ((arr[out int size], int b) in data) {}
+                Diagnostic(ErrorCode.ERR_BadArgExtraRef, "int size").WithArguments("1", "out").WithLocation(9, 27),
+                // (9,18): error CS8186: A foreach loop must declare its iteration variables.
+                //         foreach ((arr[out int size], int b) in data) {}
+                Diagnostic(ErrorCode.ERR_MustDeclareForeachIteration, "(arr[out int size], int b)").WithLocation(9, 18)
+
+                );
+        }
     }
 }