Disallow null coalescing assignment in expression trees.

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -19,7 +19,7 @@ namespace Microsoft.CodeAnalysis.CSharp {
     // class via a tool like ResGen or Visual Studio.
     // To add or remove a member, edit your .ResX file then rerun ResGen
     // with the /str option, or rebuild your VS project.
-    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "15.0.0.0")]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "16.0.0.0")]
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
     [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
     internal class CSharpResources {
@@ -4425,6 +4425,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to An expression tree may not contain a null coalescing assignment.
+        /// </summary>
+        internal static string ERR_ExpressionTreeCantContainNullCoalescingAssignment {
+            get {
+                return ResourceManager.GetString("ERR_ExpressionTreeCantContainNullCoalescingAssignment", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Expression tree cannot contain value of ref struct or restricted type &apos;{0}&apos;..
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5635,4 +5635,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ElseCannotStartStatement" xml:space="preserve">
     <value>'else' cannot start a statement.</value>
   </data>
+  <data name="ERR_ExpressionTreeCantContainNullCoalescingAssignment" xml:space="preserve">
+    <value>An expression tree may not contain a null coalescing assignment</value>
+  </data>
 </root>
\ No newline at end of file

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

@@ -1636,7 +1636,8 @@ internal enum ErrorCode
         WRN_CantInferNullabilityOfMethodTypeArgs = 8638,
         WRN_NoBestNullabilityArrayElements = 8639,
         ERR_ExpressionTreeCantContainRefStruct = 8640,
-        ERR_ElseCannotStartStatement = 8641
+        ERR_ElseCannotStartStatement = 8641,
+        ERR_ExpressionTreeCantContainNullCoalescingAssignment = 8642
 
         #endregion diagnostics introduced for C# 8.0
 

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

@@ -614,6 +614,16 @@ public override BoundNode VisitNullCoalescingOperator(BoundNullCoalescingOperato
             return base.VisitNullCoalescingOperator(node);
         }
 
+        public override BoundNode VisitNullCoalescingAssignmentOperator(BoundNullCoalescingAssignmentOperator node)
+        {
+            if (_inExpressionLambda)
+            {
+                Error(ErrorCode.ERR_ExpressionTreeCantContainNullCoalescingAssignment, node);
+            }
+
+            return base.VisitNullCoalescingAssignmentOperator(node);
+        }
+
         public override BoundNode VisitDynamicInvocation(BoundDynamicInvocation node)
         {
             if (_inExpressionLambda)

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -62,6 +62,11 @@
         <target state="new">The given expression cannot be used in a fixed statement</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ExpressionTreeCantContainNullCoalescingAssignment">
+        <source>An expression tree may not contain a null coalescing assignment</source>
+        <target state="new">An expression tree may not contain a null coalescing assignment</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExpressionTreeCantContainRefStruct">
         <source>Expression tree cannot contain value of ref struct or restricted type '{0}'.</source>
         <target state="new">Expression tree cannot contain value of ref struct or restricted type '{0}'.</target>

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenNullCoalescingAssignmentTests.cs

@@ -2351,5 +2351,23 @@ void M()
                 //         Span<byte>? s2 = null;
                 Diagnostic(ErrorCode.ERR_BadTypeArgument, "Span<byte>?").WithArguments("System.Span<byte>").WithLocation(9, 9));
         }
+
+        [Fact]
+        [WorkItem(32138, "https://github.com/dotnet/roslyn/issues/31238")]
+        public void ExpressionTreeNotAllowed()
+        {
+            CreateCompilation(@"
+using System;
+using System.Linq.Expressions;
+public class C {
+    public void M() {
+        String x = null;
+        Expression<Func<string>> e0 = () => x ??= null;
+    }
+}").VerifyDiagnostics(
+                // (7,45): error CS8642: An expression tree may not contain a null coalescing assignment
+                //         Expression<Func<string>> e0 = () => x ??= null;
+                Diagnostic(ErrorCode.ERR_ExpressionTreeCantContainNullCoalescingAssignment, "x ??= null").WithLocation(7, 45));
+        }
     }
 }