Constrains availability of ref conditional expressions to language versions 7.2 and higher

Fixes:#24306

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

@@ -3521,6 +3521,10 @@ private BoundExpression BindConditionalOperator(ConditionalExpressionSyntax node
                     diagnostics.Add(ErrorCode.ERR_RefConditionalNeedsTwoRefs, whenTrue.GetFirstToken().GetLocation());
                 }
             }
+            else
+            {
+                CheckFeatureAvailability(node, MessageID.IDS_FeatureRefConditional, diagnostics);
+            }
 
             BoundExpression condition = BindBooleanExpression(node.Condition, diagnostics);
 

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

@@ -10700,6 +10700,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to ref conditional expression.
+        /// </summary>
+        internal static string IDS_FeatureRefConditional {
+            get {
+                return ResourceManager.GetString("IDS_FeatureRefConditional", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to ref extension methods.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4264,6 +4264,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureRefStructs" xml:space="preserve">
     <value>ref structs</value>
   </data>
+  <data name="IDS_FeatureRefConditional" xml:space="preserve">
+    <value>ref conditional expression</value>
+  </data>
   <data name="CompilationC" xml:space="preserve">
     <value>Compilation (C#): </value>
   </data>

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

@@ -145,6 +145,7 @@ internal enum MessageID
         IDS_FeatureRefExtensionMethods = MessageBase + 12728,
         IDS_StackAllocExpression = MessageBase + 12729,
         IDS_FeaturePrivateProtected = MessageBase + 12730,
+        IDS_FeatureRefConditional = MessageBase + 12731,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -192,6 +193,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureRefStructs:
                 case MessageID.IDS_FeatureReadOnlyStructs:
                 case MessageID.IDS_FeatureRefExtensionMethods:
+                case MessageID.IDS_FeatureRefConditional:
                     return LanguageVersion.CSharp7_2;
 
                 // C# 7.1 features.

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

@@ -8590,6 +8590,11 @@ Pokud chcete odstranit toto varování, můžete místo toho použít /reference
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Um die Warnung zu beheben, können Sie stattdessen /reference verwenden (Einbett
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Para eliminar la advertencia puede usar /reference (establezca la propiedad Embe
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Pour supprimer l'avertissement, vous pouvez utiliser la commande /reference (dé
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Per rimuovere l'avviso, è invece possibile usare /reference (impostare la propr
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Aby usunąć ostrzeżenie, możesz zamiast tego użyć opcji /reference (ustaw w
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Para incorporar informações de tipo de interoperabilidade para os dois assembl
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ Uyarıyı kaldırmak için, /reference kullanabilirsiniz (Birlikte Çalışma T
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -8590,6 +8590,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
         <target state="new">Arguments with 'in' modifier cannot be used in dynamically dispatched expessions.</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureRefConditional">
+        <source>ref conditional expression</source>
+        <target state="new">ref conditional expression</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

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

@@ -636,6 +636,37 @@ static void Main()
                );
         }
 
+        [Fact]
+        [WorkItem(24306, "https://github.com/dotnet/roslyn/issues/24306")]
+        public void TestRefConditional_71()
+        {
+            var source = @"
+class C
+{
+    static void Main()
+    {
+
+    }
+
+    void Test()
+    {
+        int local1 = 1;
+        int local2 = 2;
+        bool b = true;
+
+        ref int r = ref b? ref local1: ref local2;
+    }
+}
+";
+            var comp = CreateCompilationWithMscorlib45(source, options: TestOptions.ReleaseExe, parseOptions:TestOptions.Regular7_1);
+
+            comp.VerifyEmitDiagnostics(
+                // (15,25): error CS8302: Feature 'ref conditional expression' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //         ref int r = ref b? ref local1: ref local2;
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "b? ref local1: ref local2").WithArguments("ref conditional expression", "7.2").WithLocation(15, 25)
+               );
+        }
+
         [Fact]
         public void TestRefConditionalUnsafeToReturn1()
         {