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Parse partial events and constructors (#76860)
* Parse partial events and constructors * Update pre-existing tests * Explain why partial ctors are sometimes disallowed * Parse `partial` constructors unconditionally * Gate parsing on LangVersion * Extend tests and docs * Fix indentation * Rename combinatorial values * Test more lang versions * Parse partial events unconditionally
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‎docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md‎

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

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

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

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

Lines changed: 20 additions & 19 deletions
Original file line numberDiff line numberDiff line change
@@ -1634,24 +1634,25 @@ private bool IsPartialType()
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private bool IsPartialMember()
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{
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// note(cyrusn): this could have been written like so:
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//
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// return
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// this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword &&
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// this.PeekToken(1).Kind == SyntaxKind.VoidKeyword;
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//
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// However, we want to be lenient and allow the user to write
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// 'partial' in most modifier lists. We will then provide them with
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// a more specific message later in binding that they are doing
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// something wrong.
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//
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// Some might argue that the simple check would suffice.
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// However, we'd like to maintain behavior with
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// previously shipped versions, and so we're keeping this code.
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Debug.Assert(this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword);
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// Check for:
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// partial event
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if (this.PeekToken(1).Kind == SyntaxKind.EventKeyword)
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{
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return true;
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}
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// Check for constructor:
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// partial Identifier(
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if (this.PeekToken(1).Kind == SyntaxKind.IdentifierToken &&
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this.PeekToken(2).Kind == SyntaxKind.OpenParenToken)
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{
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return IsFeatureEnabled(MessageID.IDS_FeaturePartialEventsAndConstructors);
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}
16511653

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// Here we check for:
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// Check for method/property:
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// partial ReturnType MemberName
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Debug.Assert(this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword);
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using var _ = this.GetDisposableResetPoint(resetOnDispose: true);
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this.EatToken(); // partial
@@ -5680,7 +5681,7 @@ private bool IsTrueIdentifier()
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{
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if (this.CurrentToken.Kind == SyntaxKind.IdentifierToken)
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{
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if (!IsCurrentTokenPartialKeywordOfPartialMethodOrType() &&
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if (!IsCurrentTokenPartialKeywordOfPartialMemberOrType() &&
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!IsCurrentTokenQueryKeywordInQuery() &&
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!IsCurrentTokenWhereOfConstraintClause())
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{
@@ -5727,7 +5728,7 @@ private SyntaxToken ParseIdentifierToken(ErrorCode code = ErrorCode.ERR_Identifi
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// show the correct parameter help in this case. So, when we see "partial" we check if it's being used
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// as an identifier or as a contextual keyword. If it's the latter then we bail out. See
57295730
// Bug: vswhidbey/542125
5730-
if (IsCurrentTokenPartialKeywordOfPartialMethodOrType() || IsCurrentTokenQueryKeywordInQuery())
5731+
if (IsCurrentTokenPartialKeywordOfPartialMemberOrType() || IsCurrentTokenQueryKeywordInQuery())
57315732
{
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var result = CreateMissingIdentifierToken();
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result = this.AddError(result, ErrorCode.ERR_InvalidExprTerm, this.CurrentToken.Text);
@@ -5754,7 +5755,7 @@ private bool IsCurrentTokenQueryKeywordInQuery()
57545755
return this.IsInQuery && this.IsCurrentTokenQueryContextualKeyword;
57555756
}
57565757

5757-
private bool IsCurrentTokenPartialKeywordOfPartialMethodOrType()
5758+
private bool IsCurrentTokenPartialKeywordOfPartialMemberOrType()
57585759
{
57595760
if (this.CurrentToken.ContextualKind == SyntaxKind.PartialKeyword)
57605761
{

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

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@@ -0,0 +1,115 @@
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// Licensed to the .NET Foundation under one or more agreements.
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// The .NET Foundation licenses this file to you under the MIT license.
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// See the LICENSE file in the project root for more information.
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using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
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using Xunit;
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namespace Microsoft.CodeAnalysis.CSharp.UnitTests;
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public sealed class PartialEventsAndConstructorsTests : CSharpTestBase
11+
{
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[Fact]
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public void ReturningPartialType_LocalFunction_InMethod()
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{
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var source = """
16+
class @partial
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{
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static void Main()
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{
20+
System.Console.Write(F().GetType().Name);
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partial F() => new();
22+
}
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}
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""";
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CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
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var expectedDiagnostics = new[]
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{
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// (5,30): error CS0103: The name 'F' does not exist in the current context
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// System.Console.Write(F().GetType().Name);
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Diagnostic(ErrorCode.ERR_NameNotInContext, "F").WithArguments("F").WithLocation(5, 30),
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// (5,50): error CS1513: } expected
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// System.Console.Write(F().GetType().Name);
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Diagnostic(ErrorCode.ERR_RbraceExpected, "").WithLocation(5, 50),
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// (6,9): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
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// partial F() => new();
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Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(6, 9),
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// (6,17): error CS1520: Method must have a return type
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// partial F() => new();
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Diagnostic(ErrorCode.ERR_MemberNeedsType, "F").WithLocation(6, 17),
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// (6,24): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
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// partial F() => new();
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Diagnostic(ErrorCode.ERR_IllegalStatement, "new()").WithLocation(6, 24),
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// (8,1): error CS1022: Type or namespace definition, or end-of-file expected
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// }
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Diagnostic(ErrorCode.ERR_EOFExpected, "}").WithLocation(8, 1)
47+
};
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CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
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CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
51+
}
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[Fact]
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public void ReturningPartialType_LocalFunction_TopLevel()
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{
56+
var source = """
57+
System.Console.Write(F().GetType().Name);
58+
partial F() => new();
59+
class @partial;
60+
""";
61+
CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
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var expectedDiagnostics = new[]
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{
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// (1,22): error CS0103: The name 'F' does not exist in the current context
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// System.Console.Write(F().GetType().Name);
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Diagnostic(ErrorCode.ERR_NameNotInContext, "F").WithArguments("F").WithLocation(1, 22),
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// (2,9): error CS0116: A namespace cannot directly contain members such as fields, methods or statements
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// partial F() => new();
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Diagnostic(ErrorCode.ERR_NamespaceUnexpected, "F").WithLocation(2, 9),
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// (2,10): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
72+
// partial F() => new();
73+
Diagnostic(ErrorCode.ERR_IllegalStatement, "() => new()").WithLocation(2, 10)
74+
};
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CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
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CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
78+
}
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[Fact]
81+
public void ReturningPartialType_Method()
82+
{
83+
var source = """
84+
class C
85+
{
86+
partial F() => new();
87+
static void Main()
88+
{
89+
System.Console.Write(new C().F().GetType().Name);
90+
}
91+
}
92+
class @partial;
93+
""";
94+
CompileAndVerify(source, parseOptions: TestOptions.Regular13, expectedOutput: "partial").VerifyDiagnostics();
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var expectedDiagnostics = new[]
97+
{
98+
// (3,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
99+
// partial F() => new();
100+
Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(3, 5),
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// (3,13): error CS1520: Method must have a return type
102+
// partial F() => new();
103+
Diagnostic(ErrorCode.ERR_MemberNeedsType, "F").WithLocation(3, 13),
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// (3,20): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement
105+
// partial F() => new();
106+
Diagnostic(ErrorCode.ERR_IllegalStatement, "new()").WithLocation(3, 20),
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// (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?)
108+
// System.Console.Write(new C().F().GetType().Name);
109+
Diagnostic(ErrorCode.ERR_NoSuchMemberOrExtension, "F").WithArguments("C", "F").WithLocation(6, 38)
110+
};
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112+
CreateCompilation(source, parseOptions: TestOptions.RegularNext).VerifyDiagnostics(expectedDiagnostics);
113+
CreateCompilation(source).VerifyDiagnostics(expectedDiagnostics);
114+
}
115+
}

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

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Original file line numberDiff line numberDiff line change
@@ -901,48 +901,30 @@ sealed static partial I3() {}
901901
targetFramework: _supportingFramework);
902902

903903
compilation1.VerifyDiagnostics(
904-
// (4,19): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
904+
// (4,19): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
905905
// sealed static partial I2();
906-
Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(4, 19),
907-
// (4,27): error CS0501: 'I2.I2()' must declare a body because it is not marked abstract, extern, or partial
906+
Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(4, 19),
907+
// (4,27): error CS0106: The modifier 'sealed' is not valid for this item
908908
// sealed static partial I2();
909-
Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I2").WithArguments("I2.I2()").WithLocation(4, 27),
910-
// (4,27): error CS0542: 'I2': member names cannot be the same as their enclosing type
911-
// sealed static partial I2();
912-
Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I2").WithArguments("I2").WithLocation(4, 27),
913-
// (9,12): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
914-
// static partial I2() {}
915-
Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(9, 12),
916-
// (9,20): error CS0542: 'I2': member names cannot be the same as their enclosing type
909+
Diagnostic(ErrorCode.ERR_BadMemberFlag, "I2").WithArguments("sealed").WithLocation(4, 27),
910+
// (9,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
917911
// static partial I2() {}
918-
Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I2").WithArguments("I2").WithLocation(9, 20),
912+
Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(9, 12),
919913
// (9,20): error CS0111: Type 'I2' already defines a member called 'I2' with the same parameter types
920914
// static partial I2() {}
921915
Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I2").WithArguments("I2", "I2").WithLocation(9, 20),
922-
// (9,20): error CS0161: 'I2.I2()': not all code paths return a value
923-
// static partial I2() {}
924-
Diagnostic(ErrorCode.ERR_ReturnExpected, "I2").WithArguments("I2.I2()").WithLocation(9, 20),
925-
// (14,12): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
916+
// (14,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
926917
// static partial I3();
927-
Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(14, 12),
928-
// (14,20): error CS0501: 'I3.I3()' must declare a body because it is not marked abstract, extern, or partial
929-
// static partial I3();
930-
Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I3").WithArguments("I3.I3()").WithLocation(14, 20),
931-
// (14,20): error CS0542: 'I3': member names cannot be the same as their enclosing type
932-
// static partial I3();
933-
Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I3").WithArguments("I3").WithLocation(14, 20),
934-
// (19,19): error CS0246: The type or namespace name 'partial' could not be found (are you missing a using directive or an assembly reference?)
918+
Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(14, 12),
919+
// (19,19): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
935920
// sealed static partial I3() {}
936-
Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "partial").WithArguments("partial").WithLocation(19, 19),
937-
// (19,27): error CS0542: 'I3': member names cannot be the same as their enclosing type
921+
Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(19, 19),
922+
// (19,27): error CS0106: The modifier 'sealed' is not valid for this item
938923
// sealed static partial I3() {}
939-
Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I3").WithArguments("I3").WithLocation(19, 27),
924+
Diagnostic(ErrorCode.ERR_BadMemberFlag, "I3").WithArguments("sealed").WithLocation(19, 27),
940925
// (19,27): error CS0111: Type 'I3' already defines a member called 'I3' with the same parameter types
941926
// sealed static partial I3() {}
942-
Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I3").WithArguments("I3", "I3").WithLocation(19, 27),
943-
// (19,27): error CS0161: 'I3.I3()': not all code paths return a value
944-
// sealed static partial I3() {}
945-
Diagnostic(ErrorCode.ERR_ReturnExpected, "I3").WithArguments("I3.I3()").WithLocation(19, 27)
927+
Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "I3").WithArguments("I3", "I3").WithLocation(19, 27)
946928
);
947929
}
948930

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