Disallow initializers on partial constructor definitions

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8065,4 +8065,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_PartialEventInitializer" xml:space="preserve">
     <value>'{0}': partial event cannot have initializer</value>
   </data>
+  <data name="ERR_PartialConstructorInitializer" xml:space="preserve">
+    <value>'{0}': only the implementing declaration of a partial constructor can have an initializer</value>
+  </data>
 </root>

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

@@ -2366,6 +2366,7 @@ internal enum ErrorCode
         ERR_PartialMemberDuplicateDefinition = 9402,
         ERR_PartialMemberDuplicateImplementation = 9403,
         ERR_PartialEventInitializer = 9404,
+        ERR_PartialConstructorInitializer = 9405,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

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

@@ -2484,6 +2484,7 @@ or ErrorCode.ERR_PartialMemberMissingDefinition
                 or ErrorCode.ERR_PartialMemberDuplicateDefinition
                 or ErrorCode.ERR_PartialMemberDuplicateImplementation
                 or ErrorCode.ERR_PartialEventInitializer
+                or ErrorCode.ERR_PartialConstructorInitializer
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

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

@@ -65,6 +65,11 @@ private SourceConstructorSymbol(
                 }
             }
 
+            if (IsPartialDefinition && syntax.Initializer is { } initializer)
+            {
+                diagnostics.Add(ErrorCode.ERR_PartialConstructorInitializer, initializer, this);
+            }
+
             if (methodKind == MethodKind.StaticConstructor)
             {
                 CheckFeatureAvailabilityAndRuntimeSupport(syntax, location, hasAnyBody, diagnostics);

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

@@ -778,6 +778,163 @@ partial event System.Action I.E { add { } remove { } }
             Diagnostic(ErrorCode.ERR_PartialMemberNotExplicit, "E").WithLocation(8, 35));
     }
 
+    [Fact]
+    public void ConstructorInitializers_This_Duplicate()
+    {
+        var source = """
+            partial class C
+            {
+                partial C() : this(1) { }
+                partial C() : this(2);
+
+                C(int x) { }
+            }
+            """;
+        CreateCompilation(source).VerifyDiagnostics(
+            // (4,17): error CS9405: 'C.C()': only the implementing declaration of a partial constructor can have an initializer
+            //     partial C() : this(2);
+            Diagnostic(ErrorCode.ERR_PartialConstructorInitializer, ": this(2)").WithArguments("C.C()").WithLocation(4, 17));
+    }
+
+    [Fact]
+    public void ConstructorInitializers_This_OnDefinition()
+    {
+        var source = """
+            partial class C
+            {
+                partial C() { }
+                partial C() : this(1);
+
+                C(int x) { }
+            }
+            """;
+        CreateCompilation(source).VerifyDiagnostics(
+            // (4,17): error CS9405: 'C.C()': only the implementing declaration of a partial constructor can have an initializer
+            //     partial C() : this(1);
+            Diagnostic(ErrorCode.ERR_PartialConstructorInitializer, ": this(1)").WithArguments("C.C()").WithLocation(4, 17));
+    }
+
+    [Fact]
+    public void ConstructorInitializers_This_OnImplementation()
+    {
+        var source = """
+            var c = new C();
+
+            partial class C
+            {
+                public partial C() : this(1) { }
+                public partial C();
+
+                C(int x) { System.Console.Write(x); }
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "1").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void ConstructorInitializers_Base_Duplicate()
+    {
+        var source = """
+            abstract class B
+            {
+                protected B(int x) { }
+            }
+
+            partial class C : B
+            {
+                partial C() : base(1) { }
+                partial C() : base(2);
+            }
+            """;
+        CreateCompilation(source).VerifyDiagnostics(
+            // (9,17): error CS9405: 'C.C()': only the implementing declaration of a partial constructor can have an initializer
+            //     partial C() : base(2);
+            Diagnostic(ErrorCode.ERR_PartialConstructorInitializer, ": base(2)").WithArguments("C.C()").WithLocation(9, 17));
+    }
+
+    [Fact]
+    public void ConstructorInitializers_Base_OnDefinition_01()
+    {
+        var source = """
+            abstract class B
+            {
+                protected B(int x) { }
+            }
+
+            partial class C : B
+            {
+                partial C() { }
+                partial C() : base(1);
+            }
+            """;
+        CreateCompilation(source).VerifyDiagnostics(
+            // (8,13): error CS7036: There is no argument given that corresponds to the required parameter 'x' of 'B.B(int)'
+            //     partial C() { }
+            Diagnostic(ErrorCode.ERR_NoCorrespondingArgument, "C").WithArguments("x", "B.B(int)").WithLocation(8, 13),
+            // (9,17): error CS9405: 'C.C()': only the implementing declaration of a partial constructor can have an initializer
+            //     partial C() : base(1);
+            Diagnostic(ErrorCode.ERR_PartialConstructorInitializer, ": base(1)").WithArguments("C.C()").WithLocation(9, 17));
+    }
+
+    [Fact]
+    public void ConstructorInitializers_Base_OnDefinition_02()
+    {
+        var source = """
+            abstract class B
+            {
+                protected B(int x) { }
+                protected B() { }
+            }
+
+            partial class C : B
+            {
+                partial C() { }
+                partial C() : base(1);
+            }
+            """;
+        CreateCompilation(source).VerifyDiagnostics(
+            // (10,17): error CS9405: 'C.C()': only the implementing declaration of a partial constructor can have an initializer
+            //     partial C() : base(1);
+            Diagnostic(ErrorCode.ERR_PartialConstructorInitializer, ": base(1)").WithArguments("C.C()").WithLocation(10, 17));
+    }
+
+    [Fact]
+    public void ConstructorInitializers_Base_OnImplementation()
+    {
+        var source = """
+            var c = new C();
+
+            abstract class B
+            {
+                protected B(int x) { System.Console.Write(x); }
+            }
+
+            partial class C : B
+            {
+                public partial C() : base(1) { }
+                public partial C();
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "1").VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void VariableInitializer()
+    {
+        var source = """
+            var c = new C();
+
+            partial class C
+            {
+                int x = 5;
+
+                public partial C() { System.Console.Write(x); }
+                public partial C();
+            }
+            """;
+        CompileAndVerify(source, expectedOutput: "5").VerifyDiagnostics();
+    }
+
     [Fact]
     public void Extern_01()
     {