Fix Local-Symbol Containing-Symbol in New-Extension-Method-Rewriter for Capture Analysis

src/Compilers/CSharp/Portable/Lowering/BoundTreeToDifferentEnclosingContextRewriter.cs

@@ -19,7 +19,7 @@ namespace Microsoft.CodeAnalysis.CSharp
     internal abstract class BoundTreeToDifferentEnclosingContextRewriter : BoundTreeRewriterWithStackGuardWithoutRecursionOnTheLeftOfBinaryOperator
     {
         // A mapping from every local variable to its replacement local variable.  Local variables are replaced when
-        // their types change due to being inside of a generic method.  Otherwise we reuse the original local (even
+        // their types change due to being inside of a generic method.  Otherwise we may reuse the original local (even
         // though its containing method is not correct because the code is moved into another method)
         private readonly Dictionary<LocalSymbol, LocalSymbol> localMap = new Dictionary<LocalSymbol, LocalSymbol>();
 
@@ -30,6 +30,8 @@ internal abstract class BoundTreeToDifferentEnclosingContextRewriter : BoundTree
 
         protected abstract MethodSymbol CurrentMethod { get; }
 
+        protected virtual bool EnforceAccurateContainerForLocals => false;
+
         public override BoundNode DefaultVisit(BoundNode node)
         {
             Debug.Fail($"Override the visitor for {node.Kind}");
@@ -57,7 +59,22 @@ protected virtual bool TryRewriteLocal(LocalSymbol local, [NotNullWhen(true)] ou
             var newType = VisitType(local.Type);
             if (TypeSymbol.Equals(newType, local.Type, TypeCompareKind.ConsiderEverything2))
             {
-                newLocal = local;
+                if (EnforceAccurateContainerForLocals)
+                {
+                    if (local.ContainingSymbol == CurrentMethod)
+                    {
+                        newLocal = local;
+                    }
+                    else
+                    {
+                        newLocal = new TypeSubstitutedLocalSymbol(local, local.TypeWithAnnotations, CurrentMethod);
+                        localMap.Add(local, newLocal);
+                    }
+                }
+                else
+                {
+                    newLocal = local;
+                }
             }
             else
             {

src/Compilers/CSharp/Portable/Lowering/ExtensionMethodBodyRewriter.cs

@@ -27,6 +27,11 @@ internal sealed class ExtensionMethodBodyRewriter : BoundTreeToDifferentEnclosin
 
         private RewrittenMethodSymbol _rewrittenContainingMethod;
 
+        /// <summary>
+        /// To allow regular capture analysis we do not want to reuse locals with an incorrect containing symbol
+        /// </summary>
+        protected override bool EnforceAccurateContainerForLocals => true;
+
         public ExtensionMethodBodyRewriter(MethodSymbol sourceMethod, SourceExtensionImplementationMethodSymbol implementationMethod)
         {
             Debug.Assert(sourceMethod is not null);