Disallow 'var' as an explicit lambda expression return type

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 6.md

@@ -108,3 +108,16 @@ These are _function_type_conversions_.
         }
     }
     ```
+
+4. In Visual Studio 17.1, the contextual keyword `var` cannot be used as an explicit lambda return type.
+
+    ```csharp
+    using System;
+
+    F(var () => default);  // error: 'var' cannot be used as an explicit lambda return type
+    F(@var () => default); // ok
+
+    static void F(Func<var> f) { }
+
+    class var { }
+    ```

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

@@ -248,6 +248,11 @@ static void checkAttributes(AnonymousFunctionExpressionSyntax syntax, SyntaxList
             MessageID.IDS_FeatureLambdaReturnType.CheckFeatureAvailability(diagnostics, syntax);
 
             syntax = syntax.SkipRef(out RefKind refKind);
+            if ((syntax as IdentifierNameSyntax)?.Identifier.ContextualKind() == SyntaxKind.VarKeyword)
+            {
+                diagnostics.Add(ErrorCode.ERR_LambdaExplicitReturnTypeVar, syntax.Location);
+            }
+
             var returnType = BindType(syntax, diagnostics);
             var type = returnType.Type;
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5845,6 +5845,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_CannotInferDelegateType" xml:space="preserve">
     <value>The delegate type could not be inferred.</value>
   </data>
+  <data name="ERR_LambdaExplicitReturnTypeVar" xml:space="preserve">
+    <value>The contextual keyword 'var' cannot be used as an explicit lambda return type</value>
+  </data>
   <data name="ERR_SingleElementPositionalPatternRequiresDisambiguation" xml:space="preserve">
     <value>A single-element deconstruct pattern requires some other syntax for disambiguation. It is recommended to add a discard designator '_' after the close paren ')'.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1999,6 +1999,7 @@ internal enum ErrorCode
 
         WRN_CompileTimeCheckedOverflow = 8973,
         WRN_MethGrpToNonDel = 8974,
+        ERR_LambdaExplicitReturnTypeVar = 8975,
 
         #endregion