Check LangVersion

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

@@ -284,6 +284,9 @@ internal enum MessageID
         IDS_FeatureParamsCollections = MessageBase + 12842,
 
         IDS_FeatureRefUnsafeInIteratorAsync = MessageBase + 12843,
+
+        // PROTOTYPE(partial-properties): pack
+        IDS_FeaturePartialProperties = MessageBase + 13000,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -468,7 +471,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureImplicitIndexerInitializer:
                 case MessageID.IDS_FeatureLockObject:
                 case MessageID.IDS_FeatureParamsCollections:
-                case MessageID.IDS_FeatureRefUnsafeInIteratorAsync:
+                case MessageID.IDS_FeaturePartialProperties:
                     return LanguageVersion.Preview;
 
                 // C# 12.0 features.

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs

@@ -384,6 +384,18 @@ private static (DeclarationModifiers modifiers, bool hasExplicitAccessMod) MakeM
                 hasExplicitAccessMod = true;
             }
 
+            if ((mods & DeclarationModifiers.Partial) != 0)
+            {
+                Debug.Assert(location.SourceTree is not null);
+
+                LanguageVersion availableVersion = ((CSharpParseOptions)location.SourceTree.Options).LanguageVersion;
+                LanguageVersion requiredVersion = MessageID.IDS_FeaturePartialProperties.RequiredVersion();
+                if (availableVersion < requiredVersion)
+                {
+                    ModifierUtils.ReportUnsupportedModifiersForLanguageVersion(mods, DeclarationModifiers.Partial, location, diagnostics, availableVersion, requiredVersion);
+                }
+            }
+
             ModifierUtils.CheckFeatureAvailabilityForStaticAbstractMembersInInterfacesIfNeeded(mods, isExplicitInterfaceImplementation, location, diagnostics);
 
             containingType.CheckUnsafeModifier(mods, location, diagnostics);

src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs

@@ -785,7 +785,7 @@ public extern static partial int F
     }
 }
 """;
-            CompileAndVerify(source, parseOptions: TestOptions.Regular.WithNoRefSafetyRulesAttribute(), assemblyValidator: (assembly) =>
+            CompileAndVerify(source, parseOptions: TestOptions.RegularPreview.WithNoRefSafetyRulesAttribute(), assemblyValidator: (assembly) =>
             {
                 var metadataReader = assembly.GetMetadataReader();
 
@@ -4410,6 +4410,39 @@ public int P
             Assert.Empty(symbolInfo.CandidateSymbols);
         }
 
+        [Fact]
+        public void LangVersion_01()
+        {
+            var source = """
+                partial class C
+                {
+                    public partial int P { get; set; }
+                    public partial int P { get => 1; set { } }
+
+                    public partial int this[int i] { get; }
+                    public partial int this[int i] { get => i; }
+                }
+                """;
+
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics();
+
+            comp = CreateCompilation(source, parseOptions: TestOptions.Regular12);
+            comp.VerifyEmitDiagnostics(
+                // (3,24): error CS8703: The modifier 'partial' is not valid for this item in C# 12.0. Please use language version 'preview' or greater.
+                //     public partial int P { get; set; }
+                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P").WithArguments("partial", "12.0", "preview").WithLocation(3, 24),
+                // (4,24): error CS8703: The modifier 'partial' is not valid for this item in C# 12.0. Please use language version 'preview' or greater.
+                //     public partial int P { get => 1; set { } }
+                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P").WithArguments("partial", "12.0", "preview").WithLocation(4, 24),
+                // (6,24): error CS8703: The modifier 'partial' is not valid for this item in C# 12.0. Please use language version 'preview' or greater.
+                //     public partial int this[int i] { get; }
+                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("partial", "12.0", "preview").WithLocation(6, 24),
+                // (7,24): error CS8703: The modifier 'partial' is not valid for this item in C# 12.0. Please use language version 'preview' or greater.
+                //     public partial int this[int i] { get => i; }
+                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("partial", "12.0", "preview").WithLocation(7, 24));
+        }
+
         // PROTOTYPE(partial-properties): override partial property where base has modopt
     }
 }