DRAFT prototype for 16195

Possible approach for fixing dotnet#16195

src/Compilers/CSharp/Portable/Binder/ExpressionVariableFinder.cs

@@ -41,6 +41,7 @@ internal abstract class ExpressionVariableFinder<TFieldOrLocalSymbol> : CSharpSy
                 case SyntaxKind.IfStatement:
                 case SyntaxKind.SwitchStatement:
                 case SyntaxKind.VariableDeclarator:
+                case SyntaxKind.LocalFunctionStatement:
                     break;
                 case SyntaxKind.ArgumentList:
                     Debug.Assert(node.Parent is ConstructorInitializerSyntax);
@@ -179,6 +180,18 @@ public override void VisitSwitchStatement(SwitchStatementSyntax node)
             VisitNodeToBind(node.Expression);
         }
 
+        public override void VisitLocalFunctionStatement(LocalFunctionStatementSyntax node)
+        {
+            // Expression variables may not be declared in a local function's
+            // parameter initializer, because the initializer is required to be a kind
+            // of constant. Although it is an error, we must handle such code.
+            // We give such variables the scope of the enclosing block.
+            foreach (var parameter in node.ParameterList.Parameters)
+            {
+                VisitNodeToBind(parameter.Default?.Value);
+            }
+        }
+
         public override void VisitDeclarationPattern(DeclarationPatternSyntax node)
         {
             var variable = MakePatternVariable(node, _nodeToBind);

src/Compilers/CSharp/Portable/Binder/LocalScopeBinder.cs

@@ -186,6 +186,7 @@ protected ImmutableArray<LocalSymbol> BuildLocals(SyntaxList<StatementSyntax> st
                         }
                         break;
 
+                    case SyntaxKind.LocalFunctionStatement:
                     case SyntaxKind.ExpressionStatement:
                     case SyntaxKind.IfStatement:
                     case SyntaxKind.YieldReturnStatement:

src/Compilers/CSharp/Test/Semantic/CSharpCompilerSemanticTest.csproj

@@ -73,6 +73,7 @@
     <Compile Include="FlowAnalysis\RegionAnalysisTests.cs" />
     <Compile Include="FlowAnalysis\StructTests.cs" />
     <Compile Include="FlowAnalysis\TryLockUsingStatementTests.cs" />
+    <Compile Include="Semantics\FuzzTests.cs" />
     <Compile Include="Semantics\PatternMatchingTests_Global.cs" />
     <Compile Include="Semantics\BindingAsyncTasklikeMoreTests.cs" />
     <Compile Include="Semantics\BindingAsyncTasklikeTests.cs" />

src/Compilers/CSharp/Test/Semantic/Semantics/FuzzTests.cs

@@ -0,0 +1,77 @@
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics
+{
+    public class FuzzTests : CompilingTestBase
+    {
+        [Fact, WorkItem(16167, "https://github.com/dotnet/roslyn/issues/16167")]
+        public void CompileXmlAsSource()
+        {
+            var text = @"
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+class C
+{
+    public static void Main()
+    {
+      <summary>Copies a range of elements from an <see cref=""T:System.Array"" /> starting at the first element and pastes them into another <see cref=""T:System.Array"" /> starting at the first element. The length is specified as a 32-bit integer.</summary>
+      <param name=""sourceArray"">The <see cref=""T:System.Array"" /> that contains the data to copy.</param>
+      <param name=""destinationArray"">The <see cref=""T:System.Array"" /> that receives the data.</param>
+      <param name=""length"">A 32-bit integer that represents the number of elements to copy.</param>
+      <exception cref=""T:System.ArgumentNullException"">
+        <paramref name=""sourceArray"" /> is null.-or-<paramref name=""destinationArray"" /> is null.</exception>
+      <exception cref=""T:System.RankException"">
+        <paramref name=""sourceArray"" /> and <paramref name=""destinationArray"" /> have different ranks.</exception>
+      <exception cref=""T:System.ArrayTypeMismatchException"">
+        <paramref name=""sourceArray"" /> and <paramref name=""destinationArray"" /> are of incompatible types.</exception>
+      <exception cref=""T:System.InvalidCastException"">At least one element in <paramref name=""sourceArray"" /> cannot be cast to the type of <paramref name=""destinationArray"" />.</exception>
+      <exception cref=""T:System.ArgumentOutOfRangeException"">
+        <paramref name=""length"" /> is less than zero.</exception>
+      <exception cref=""T:System.ArgumentException"">
+        <paramref name=""length"" /> is greater than the number of elements in <paramref name=""sourceArray"" />.-or-<paramref name=""length"" /> is greater than the number of elements in <paramref name=""destinationArray"" />.</exception>
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(text);
+            var tree = compilation.SyntaxTrees[0];
+            var model = compilation.GetSemanticModel(tree);
+            foreach (var node in tree.GetRoot().DescendantNodes())
+            {
+                var _ = model.GetSymbolInfo(node);
+            }
+        }
+
+        [Fact, WorkItem(16167, "https://github.com/dotnet/roslyn/issues/16167")]
+        public void Bug16167Root()
+        {
+            var text = @"
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+class C
+{
+    public static void Main(string[] args)
+    {
+        void Local1(bool b = args is string[] z1) {}
+        void Local2(bool b = M(out int z2)) {}
+        void Local3((int, int) ii = ((int a1, int a2) = (1, 2)) {}
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(text);
+            var tree = compilation.SyntaxTrees[0];
+            var model = compilation.GetSemanticModel(tree);
+            foreach (var node in tree.GetRoot().DescendantNodes())
+            {
+                var _ = model.GetSymbolInfo(node);
+            }
+        }
+    }
+}