Merge pull request #2871 from AlekseyTs/Issue2862

Do not hoist ByRef locals used within iterator methods, even when building debug.

src/Compilers/VisualBasic/Portable/Analysis/IteratorAndAsyncAnalysis/IteratorAndAsyncCaptureWalker.vb

@@ -45,6 +45,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
         ' Returns deterministically ordered list of variables that ought to be hoisted.
         Public Overloads Shared Function Analyze(info As FlowAnalysisInfo, diagnostics As DiagnosticBag) As Result
+            Debug.Assert(info.Symbol IsNot Nothing)
+            Debug.Assert(info.Symbol.Kind = SymbolKind.Method)
+
             Dim walker As New IteratorAndAsyncCaptureWalker(info)
             walker.Analyze()
             Debug.Assert(Not walker.InvalidRegionDetected)
@@ -69,16 +72,21 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             If info.Compilation.Options.OptimizationLevel <> OptimizationLevel.Release Then
                 Debug.Assert(variablesToHoist.Count = 0)
 
-                ' In debug build we hoist all locals and parameters:
-                variablesToHoist.AddRange(From v In allVariables
-                                          Where v.Symbol IsNot Nothing AndAlso HoistInDebugBuild(v.Symbol)
-                                          Select v.Symbol)
+                ' In debug build we hoist all locals and parameters, except ByRef locals in iterator methods.
+                ' Lifetime of ByRef locals in iterator methods never crosses stament boundaries, thus 
+                ' there is no reason to hoist them and the pipeline doesn't handle this.
+                Dim skipByRefLocals As Boolean = DirectCast(info.Symbol, MethodSymbol).IsIterator
+                For Each v In allVariables
+                    If v.Symbol IsNot Nothing AndAlso HoistInDebugBuild(v.Symbol, skipByRefLocals) Then
+                        variablesToHoist.Add(v.Symbol)
+                    End If
+                Next
             End If
 
             Return New Result(variablesToHoist, byRefLocalsInitializers)
         End Function
 
-        Private Shared Function HoistInDebugBuild(symbol As Symbol) As Boolean
+        Private Shared Function HoistInDebugBuild(symbol As Symbol, skipByRefLocals As Boolean) As Boolean
             ' In debug build hoist all parameters that can be hoisted:
             If symbol.Kind = SymbolKind.Parameter Then
                 Dim parameter = TryCast(symbol, ParameterSymbol)
@@ -91,6 +99,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                     Return False
                 End If
 
+                If skipByRefLocals AndAlso local.IsByRef Then
+                    Return False
+                End If
+
                 ' hoist all user-defined locals that can be hoisted
                 If local.SynthesizedKind = SynthesizedLocalKind.UserDefined Then
                     Return Not local.Type.IsRestrictedType()

src/Compilers/VisualBasic/Test/Semantic/Semantics/IteratorTests.vb

@@ -936,6 +936,61 @@ BC36947: Single-line lambdas cannot have the 'Iterator' modifier. Use a multilin
 </errors>)
         End Sub
 
+        <Fact(), WorkItem(1173145, "DevDiv"), WorkItem(2862, "https://github.com/dotnet/roslyn/issues/2862")>
+        Public Sub CompoundAssignmentToAField()
+            Dim compilation = CreateCompilationWithMscorlibAndVBRuntime(
+                    <compilation>
+                        <file name="a.vb">
+                            <![CDATA[
+Imports System.Collections 
+Imports System.Collections.Generic 
+
+Module Module1
+    Sub Main()
+        For Each x In New MyEnumerable(Of Integer)({100, 99, 98})
+            System.Console.WriteLine(x)
+        Next
+    End Sub
+End Module
+
+Public Class MyEnumerable(Of T)
+    Implements IEnumerable(Of T)
+
+    Private ReadOnly _items As T()
+    Private _count As Integer = 0
+
+    Public Sub New(items As T())
+        _items = items
+        _count = items.Length
+    End Sub
+
+    Public Iterator Function GetEnumerator() As IEnumerator(Of T) Implements IEnumerable(Of T).GetEnumerator
+        For Each item In _items
+            If _count = 0 Then Exit Function
+            _count -= 1
+            Yield item
+        Next
+    End Function
+
+    Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
+        Return GetEnumerator()
+    End Function
+End Class
+]]>
+                        </file>
+                    </compilation>, TestOptions.DebugExe)
+
+            Dim expected As Xml.Linq.XCData = <![CDATA[
+100
+99
+98
+]]>
+
+            CompileAndVerify(compilation, expected)
+
+            CompileAndVerify(compilation.WithOptions(TestOptions.ReleaseExe), expected)
+        End Sub
+
     End Class
 
 End Namespace