Fix out var behavior, including with NotNullWhenTrue/False

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

@@ -374,7 +374,7 @@ private void FailRemainingInferencesAndSetValEscape(ArrayBuilder<DeconstructionV
                     {
                         case BoundKind.Local:
                             var local = (BoundLocal)variable.Single;
-                            if (local.IsDeclaration)
+                            if (local.DeclarationKind != BoundLocalDeclarationKind.None)
                             {
                                 ((SourceLocalSymbol)local.LocalSymbol).SetValEscape(rhsValEscape);
                             }
@@ -826,7 +826,7 @@ private BoundExpression BindDeconstructionVariable(
 
                 if ((object)declType != null)
                 {
-                    return new BoundLocal(syntax, localSymbol, isDeclaration: true, constantValueOpt: null, isNullableUnknown: false, type: declType.TypeSymbol, hasErrors: hasErrors);
+                    return new BoundLocal(syntax, localSymbol, BoundLocalDeclarationKind.WithExplicitType, constantValueOpt: null, isNullableUnknown: false, type: declType.TypeSymbol, hasErrors: hasErrors);
                 }
 
                 return new DeconstructionVariablePendingInference(syntax, localSymbol, receiverOpt: null);

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

@@ -1549,7 +1549,7 @@ private BoundExpression BindNonMethod(SimpleNameSyntax node, Symbol symbol, Diag
 
                         var constantValueOpt = localSymbol.IsConst && !IsInsideNameof && !type.IsErrorType()
                             ? localSymbol.GetConstantValue(node, this.LocalInProgress, diagnostics) : null;
-                        return new BoundLocal(node, localSymbol, isDeclaration: false, constantValueOpt: constantValueOpt, isNullableUnknown: isNullableUnknown, type: type, hasErrors: isError);
+                        return new BoundLocal(node, localSymbol, BoundLocalDeclarationKind.None, constantValueOpt: constantValueOpt, isNullableUnknown: isNullableUnknown, type: type, hasErrors: isError);
                     }
 
                 case SymbolKind.Parameter:
@@ -2347,7 +2347,7 @@ private BoundExpression BindOutVariableDeclarationArgument(
 
                 CheckRestrictedTypeInAsync(this.ContainingMemberOrLambda, declType.TypeSymbol, diagnostics, typeSyntax);
 
-                return new BoundLocal(declarationExpression, localSymbol, isDeclaration: true, constantValueOpt: null, isNullableUnknown: false, type: declType.TypeSymbol);
+                return new BoundLocal(declarationExpression, localSymbol, BoundLocalDeclarationKind.WithExplicitType, constantValueOpt: null, isNullableUnknown: false, type: declType.TypeSymbol);
             }
 
             // Is this a field?

src/Compilers/CSharp/Portable/BoundTree/BoundExpression.cs

@@ -133,13 +133,13 @@ public override Symbol ExpressionSymbol
         }
 
         public BoundLocal(SyntaxNode syntax, LocalSymbol localSymbol, ConstantValue constantValueOpt, TypeSymbol type, bool hasErrors = false)
-            : this(syntax, localSymbol, false, constantValueOpt, false, type, hasErrors)
+            : this(syntax, localSymbol, BoundLocalDeclarationKind.None, constantValueOpt, false, type, hasErrors)
         {
         }
 
         public BoundLocal Update(LocalSymbol localSymbol, ConstantValue constantValueOpt, TypeSymbol type)
         {
-            return this.Update(localSymbol, this.IsDeclaration, constantValueOpt, this.IsNullableUnknown, type);
+            return this.Update(localSymbol, this.DeclarationKind, constantValueOpt, this.IsNullableUnknown, type);
         }
     }
 

src/Compilers/CSharp/Portable/BoundTree/BoundLocalDeclarationKind.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+namespace Microsoft.CodeAnalysis.CSharp
+{
+    /// <summary>
+    /// Indicates whether a bound local is also a declaration, and if so was it a declaration with an explicit or an inferred type.
+    /// Ex:
+    /// - In `M(x)`, `x` has `LocalDeclarationKind.None`
+    /// - In `M(out int x)`, `x` has `LocalDeclarationKind.WithExplicitType`
+    /// - In `M(out var x)`, `x` has `LocalDeclarationKind.WithInferredType`
+    /// </summary>
+    internal enum BoundLocalDeclarationKind
+    {
+        None = 0,
+        WithExplicitType,
+        WithInferredType
+    }
+}

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -30,6 +30,7 @@
   <ValueType Name="NoOpStatementFlavor"/>
   <ValueType Name="RefKind"/>
   <ValueType Name="BoundTypeOrValueData"/>
+  <ValueType Name="BoundLocalDeclarationKind"/>
 
   <AbstractNode Name="BoundInitializer" Base="BoundNode"/>
 
@@ -1059,7 +1060,7 @@
     <!-- Non-null type is required for this node kind -->
     <Field Name="Type" Type="TypeSymbol" Override="true" Null="disallow"/>
     <Field Name="LocalSymbol" Type="LocalSymbol"/>
-    <Field Name="IsDeclaration" Type="bool"/>
+    <Field Name="DeclarationKind" Type="BoundLocalDeclarationKind" Null="NotApplicable"/>
     <Field Name="ConstantValueOpt" Type="ConstantValue" Null="allow"/>
     <!-- True if LocalSymbol.Type.IsNullable could not be inferred. -->
     <Field Name="IsNullableUnknown" Type="bool"/>
@@ -1728,7 +1729,7 @@
   <!-- Special node, used only during nullability flow analysis -->
   <!-- PROTOTYPE(NullableReferenceTypes): Derive from BoundValuePlaceholderBase and give specific name. -->
   <Node Name="BoundValuePlaceholder" Base="BoundExpression">
-    <Field Name="Type" Type="TypeSymbol" Override="true" Null="disallow"/>
+    <Field Name="Type" Type="TypeSymbol" Override="true" Null="allow"/> <!-- We use null Type for placeholders representing out vars -->
     <Field Name="IsNullable" Type="bool?" Null="allow"/>
   </Node>
 </Tree>

src/Compilers/CSharp/Portable/BoundTree/VariablePendingInference.cs

@@ -63,7 +63,7 @@ internal BoundExpression SetInferredType(TypeSymbol type, Binder binderOpt, Diag
                     }
 
                     localSymbol.SetTypeSymbol(TypeSymbolWithAnnotations.Create(type));
-                    return new BoundLocal(this.Syntax, localSymbol, isDeclaration: true, constantValueOpt: null, isNullableUnknown: false, type: type, hasErrors: this.HasErrors || inferenceFailed);
+                    return new BoundLocal(this.Syntax, localSymbol, BoundLocalDeclarationKind.WithInferredType, constantValueOpt: null, isNullableUnknown: false, type: type, hasErrors: this.HasErrors || inferenceFailed);
 
                 case SymbolKind.Field:
                     var fieldSymbol = (GlobalExpressionVariable)this.VariableSymbol;

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

@@ -51,7 +51,7 @@ public override bool Equals(object obj)
 
             internal string GetDebuggerDisplay()
             {
-                return $"ContainingSlot={ContainingSlot}, Symbol={Symbol}";
+                return $"ContainingSlot={ContainingSlot}, Symbol={Symbol.GetDebuggerDisplay()}";
             }
         }
     }