Parse partial events and constructors

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md

@@ -291,3 +291,20 @@ unsafe record struct R(
     public bool Equals(R other) => true;
 }
 ```
+
+## `partial` cannot be a return type of methods
+
+***Introduced in Visual Studio 2022 version 17.14***
+
+The [partial events and constructors](https://github.com/dotnet/csharplang/issues/9058) language feature
+allows the `partial` modifier in more places and so it cannot be a return type unless escaped:
+
+```cs
+class C
+{
+    partial F() => new partial(); // previously worked
+    @partial F() => new partial(); // workaround
+}
+
+class partial { }
+```

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

@@ -296,6 +296,8 @@ internal enum MessageID
         IDS_FeatureFirstClassSpan = MessageBase + 12849,
         IDS_FeatureUnboundGenericTypesInNameof = MessageBase + 12850,
         IDS_FeatureSimpleLambdaParameterModifiers = MessageBase + 12851,
+
+        IDS_FeaturePartialEventsAndConstructors = MessageBase + 12852,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -480,6 +482,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureFirstClassSpan:
                 case MessageID.IDS_FeatureUnboundGenericTypesInNameof:
                 case MessageID.IDS_FeatureSimpleLambdaParameterModifiers:
+                case MessageID.IDS_FeaturePartialEventsAndConstructors:
                     return LanguageVersion.Preview;
 
                 // C# 13.0 features.

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -1634,24 +1634,25 @@ private bool IsPartialType()
 
         private bool IsPartialMember()
         {
-            // note(cyrusn): this could have been written like so:
-            //
-            //  return
-            //    this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword &&
-            //    this.PeekToken(1).Kind == SyntaxKind.VoidKeyword;
-            //
-            // However, we want to be lenient and allow the user to write 
-            // 'partial' in most modifier lists.  We will then provide them with
-            // a more specific message later in binding that they are doing 
-            // something wrong.
-            //
-            // Some might argue that the simple check would suffice.
-            // However, we'd like to maintain behavior with 
-            // previously shipped versions, and so we're keeping this code.
+            Debug.Assert(this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword);
 
-            // Here we check for:
+            // Check for:
+            //   partial event
+            if (this.PeekToken(1).Kind == SyntaxKind.EventKeyword)
+            {
+                return IsFeatureEnabled(MessageID.IDS_FeaturePartialEventsAndConstructors);
+            }
+
+            // Check for constructor:
+            //   partial Identifier(
+            if (this.PeekToken(1).Kind == SyntaxKind.IdentifierToken &&
+                this.PeekToken(2).Kind == SyntaxKind.OpenParenToken)
+            {
+                return IsFeatureEnabled(MessageID.IDS_FeaturePartialEventsAndConstructors);
+            }
+
+            // Check for method/property:
             //   partial ReturnType MemberName
-            Debug.Assert(this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword);
             using var _ = this.GetDisposableResetPoint(resetOnDispose: true);
 
             this.EatToken(); // partial
@@ -5680,7 +5681,7 @@ private bool IsTrueIdentifier()
         {
             if (this.CurrentToken.Kind == SyntaxKind.IdentifierToken)
             {
-                if (!IsCurrentTokenPartialKeywordOfPartialMethodOrType() &&
+                if (!IsCurrentTokenPartialKeywordOfPartialMemberOrType() &&
                     !IsCurrentTokenQueryKeywordInQuery() &&
                     !IsCurrentTokenWhereOfConstraintClause())
                 {
@@ -5727,7 +5728,7 @@ private SyntaxToken ParseIdentifierToken(ErrorCode code = ErrorCode.ERR_Identifi
                 // show the correct parameter help in this case.  So, when we see "partial" we check if it's being used
                 // as an identifier or as a contextual keyword.  If it's the latter then we bail out.  See
                 // Bug: vswhidbey/542125
-                if (IsCurrentTokenPartialKeywordOfPartialMethodOrType() || IsCurrentTokenQueryKeywordInQuery())
+                if (IsCurrentTokenPartialKeywordOfPartialMemberOrType() || IsCurrentTokenQueryKeywordInQuery())
                 {
                     var result = CreateMissingIdentifierToken();
                     result = this.AddError(result, ErrorCode.ERR_InvalidExprTerm, this.CurrentToken.Text);
@@ -5754,7 +5755,7 @@ private bool IsCurrentTokenQueryKeywordInQuery()
             return this.IsInQuery && this.IsCurrentTokenQueryContextualKeyword;
         }
 
-        private bool IsCurrentTokenPartialKeywordOfPartialMethodOrType()
+        private bool IsCurrentTokenPartialKeywordOfPartialMemberOrType()
         {
             if (this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword)
             {

src/Compilers/CSharp/Test/Emit3/PartialEventsAndConstructorsTests.cs

@@ -0,0 +1,115 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests;
+
+public sealed class PartialEventsAndConstructorsTests : CSharpTestBase
+{
+    [Fact]
+    public void ReturningPartialType_LocalFunction_InMethod()
+    {
+        var source = """
+            class @partial
+            {
+                static void Main()
+                {
+                    System.Console.Write(F().GetType().Name);
+                    partial F() => new();
+                }
+            }
+            """;
+        CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
+
+        var expectedDiagnostics = new[]
+        {
+            // (5,30): error CS0103: The name 'F' does not exist in the current context
+            //         System.Console.Write(F().GetType().Name);
+            Diagnostic(ErrorCode.ERR_NameNotInContext, "F").WithArguments("F").WithLocation(5, 30),
+            // (5,50): error CS1513: } expected
+            //         System.Console.Write(F().GetType().Name);
+            Diagnostic(ErrorCode.ERR_RbraceExpected, "").WithLocation(5, 50),
+            // (6,9): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
+            //         partial F() => new();
+            Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(6, 9),
+            // (6,17): error CS1520: Method must have a return type
+            //         partial F() => new();
+            Diagnostic(ErrorCode.ERR_MemberNeedsType, "F").WithLocation(6, 17),
+            // (6,24): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
+            //         partial F() => new();
+            Diagnostic(ErrorCode.ERR_IllegalStatement, "new()").WithLocation(6, 24),
+            // (8,1): error CS1022: Type or namespace definition, or end-of-file expected
+            // }
+            Diagnostic(ErrorCode.ERR_EOFExpected, "}").WithLocation(8, 1)
+        };
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+        CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
+    }
+
+    [Fact]
+    public void ReturningPartialType_LocalFunction_TopLevel()
+    {
+        var source = """
+            System.Console.Write(F().GetType().Name);
+            partial F() => new();
+            class @partial;
+            """;
+        CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
+
+        var expectedDiagnostics = new[]
+        {
+            // (1,22): error CS0103: The name 'F' does not exist in the current context
+            // System.Console.Write(F().GetType().Name);
+            Diagnostic(ErrorCode.ERR_NameNotInContext, "F").WithArguments("F").WithLocation(1, 22),
+            // (2,9): error CS0116: A namespace cannot directly contain members such as fields, methods or statements
+            // partial F() => new();
+            Diagnostic(ErrorCode.ERR_NamespaceUnexpected, "F").WithLocation(2, 9),
+            // (2,10): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
+            // partial F() => new();
+            Diagnostic(ErrorCode.ERR_IllegalStatement, "() => new()").WithLocation(2, 10)
+        };
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+        CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
+    }
+
+    [Fact]
+    public void ReturningPartialType_Method()
+    {
+        var source = """
+            class C
+            {
+                partial F() => new();
+                static void Main()
+                {
+                    System.Console.Write(new C().F().GetType().Name);
+                }
+            }
+            class @partial;
+            """;
+        CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
+
+        var expectedDiagnostics = new[]
+        {
+            // (3,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
+            //     partial F() => new();
+            Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(3, 5),
+            // (3,13): error CS1520: Method must have a return type
+            //     partial F() => new();
+            Diagnostic(ErrorCode.ERR_MemberNeedsType, "F").WithLocation(3, 13),
+            // (3,20): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
+            //     partial F() => new();
+            Diagnostic(ErrorCode.ERR_IllegalStatement, "new()").WithLocation(3, 20),
+            // (6,38): error CS1061: 'C' does not contain a definition for 'F' and no accessible extension method 'F' accepting a first argument of type 'C' could be found (are you missing a using directive or an assembly reference?)
+            //         System.Console.Write(new C().F().GetType().Name);
+            Diagnostic(ErrorCode.ERR_NoSuchMemberOrExtension, "F").WithArguments("C", "F").WithLocation(6, 38)
+        };
+
+        CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
+        CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
+    }
+}

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

@@ -901,48 +901,30 @@ sealed static partial I3() {}
                                                  targetFramework: _supportingFramework);
 
             compilation1.VerifyDiagnostics(
-                // (4,19): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
+                // (4,19): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
                 //     sealed static partial I2();
-                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(4, 19),
-                // (4,27): error CS0501: 'I2.I2()' must declare a body because it is not marked abstract, extern, or partial
+                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(4, 19),
+                // (4,27): error CS0106: The modifier 'sealed' is not valid for this item
                 //     sealed static partial I2();
-                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I2").WithArguments("I2.I2()").WithLocation(4, 27),
-                // (4,27): error CS0542: 'I2': member names cannot be the same as their enclosing type
-                //     sealed static partial I2();
-                Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I2").WithArguments("I2").WithLocation(4, 27),
-                // (9,12): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
-                //     static partial I2() {}
-                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(9, 12),
-                // (9,20): error CS0542: 'I2': member names cannot be the same as their enclosing type
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I2").WithArguments("sealed").WithLocation(4, 27),
+                // (9,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
                 //     static partial I2() {}
-                Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I2").WithArguments("I2").WithLocation(9, 20),
+                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(9, 12),
                 // (9,20): error CS0111: Type 'I2' already defines a member called 'I2' with the same parameter types
                 //     static partial I2() {}
                 Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I2").WithArguments("I2", "I2").WithLocation(9, 20),
-                // (9,20): error CS0161: 'I2.I2()': not all code paths return a value
-                //     static partial I2() {}
-                Diagnostic(ErrorCode.ERR_ReturnExpected, "I2").WithArguments("I2.I2()").WithLocation(9, 20),
-                // (14,12): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
+                // (14,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
                 //     static partial I3();
-                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(14, 12),
-                // (14,20): error CS0501: 'I3.I3()' must declare a body because it is not marked abstract, extern, or partial
-                //     static partial I3();
-                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I3").WithArguments("I3.I3()").WithLocation(14, 20),
-                // (14,20): error CS0542: 'I3': member names cannot be the same as their enclosing type
-                //     static partial I3();
-                Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I3").WithArguments("I3").WithLocation(14, 20),
-                // (19,19): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
+                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(14, 12),
+                // (19,19): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
                 //     sealed static partial I3() {}
-                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(19, 19),
-                // (19,27): error CS0542: 'I3': member names cannot be the same as their enclosing type
+                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(19, 19),
+                // (19,27): error CS0106: The modifier 'sealed' is not valid for this item
                 //     sealed static partial I3() {}
-                Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I3").WithArguments("I3").WithLocation(19, 27),
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I3").WithArguments("sealed").WithLocation(19, 27),
                 // (19,27): error CS0111: Type 'I3' already defines a member called 'I3' with the same parameter types
                 //     sealed static partial I3() {}
-                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I3").WithArguments("I3", "I3").WithLocation(19, 27),
-                // (19,27): error CS0161: 'I3.I3()': not all code paths return a value
-                //     sealed static partial I3() {}
-                Diagnostic(ErrorCode.ERR_ReturnExpected, "I3").WithArguments("I3.I3()").WithLocation(19, 27)
+                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I3").WithArguments("I3", "I3").WithLocation(19, 27)
                 );
         }