Make partial interface members virtual (#77379)

* Make partial interface members virtual

* Simplify code

* Document the breaking change

* Add more tests

* Gate on lang version

* Improve code and doc

* Improve wording

* Update based on LDM

* Partial methods were always implicitly private

* Revert an unrelated change

Likely caused by some unintentional git operation.

* Fix wording in doc example

* Clarify workaround

Author: jjonescz

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

@@ -364,3 +364,41 @@ class extension { } // type may not be named "extension"
 class C<extension> { } // type parameter may not be named "extension"
 ```
 
+## Partial properties and events are now implicitly virtual and public
+
+***Introduced in Visual Studio 2022 version 17.15***
+
+We have fixed [an inconsistency](https://github.com/dotnet/roslyn/issues/77346)
+where partial interface properties and events would not be implicitly `virtual` and `public` unlike their non-partial equivalents.
+This inconsistency is however [preserved](./Deviations%20from%20Standard.md#interface-partial-methods) for partial interface methods to avoid a larger breaking change.
+Note that Visual Basic and other languages not supporting default interface members will start requiring to implement implicitly virtual `partial` interface members.
+
+To keep the previous behavior, explicitly mark `partial` interface members as `private` (if they don't have any accessibility modifiers)
+and `sealed` (if they don't have the `private` modifier which implies `sealed`, and they don't already have modifier `virtual` or `sealed`).
+
+```cs
+System.Console.Write(((I)new C()).P); // wrote 1 previously, writes 2 now
+
+partial interface I
+{
+    public partial int P { get; }
+    public partial int P => 1; // implicitly virtual now
+}
+
+class C : I
+{
+    public int P => 2; // implements I.P
+}
+```
+
+```cs
+System.Console.Write(((I)new C()).P); // inaccessible previously, writes 1 now
+
+partial interface I
+{
+    partial int P { get; } // implicitly public now
+    partial int P => 1;
+}
+
+class C : I;
+```

docs/compilers/CSharp/Deviations from Standard.md

@@ -77,3 +77,10 @@ The compiler is free to make assumptions about the shape and behavior of well-kn
 It may not check for unexpected constraints, `Obsolete` attribute, or `UnmanagedCallersOnly` attribute.
 It may perform some optimizations based on expectations that the types/members are well-behaved.
 Note: the compiler should remain resilient to missing well-known types/members.
+
+# Interface partial methods
+
+Interface partial methods are implicitly non-virtual,
+unlike non-partial interface methods and other interface partial member kinds,
+see [a related breaking change](./Compiler%20Breaking%20Changes%20-%20DotNet%2010.md#partial-properties-and-events-are-now-implicitly-virtual-and-public)
+and [LDM 2025-04-07](https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-04-07.md#breaking-change-discussion-making-partial-members-in-interfaces-virtual-andor-public).

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

@@ -245,8 +245,24 @@ internal static void CheckFeatureAvailabilityForPartialEventsAndConstructors(Loc
         }
 #nullable disable
 
-        internal static DeclarationModifiers AdjustModifiersForAnInterfaceMember(DeclarationModifiers mods, bool hasBody, bool isExplicitInterfaceImplementation)
+        internal static DeclarationModifiers AdjustModifiersForAnInterfaceMember(DeclarationModifiers mods, bool hasBody, bool isExplicitInterfaceImplementation, bool forMethod)
         {
+            // Interface partial non-method members are implicitly public and virtual just like their non-partial counterparts.
+            // Interface partial methods are implicitly private and not virtual (this is a spec violation but being preserved to avoid breaks).
+            bool notPartialOrNewPartialBehavior = (mods & DeclarationModifiers.Partial) == 0 || !forMethod;
+
+            if ((mods & DeclarationModifiers.AccessibilityMask) == 0)
+            {
+                if (!isExplicitInterfaceImplementation && notPartialOrNewPartialBehavior)
+                {
+                    mods |= DeclarationModifiers.Public;
+                }
+                else
+                {
+                    mods |= DeclarationModifiers.Private;
+                }
+            }
+
             if (isExplicitInterfaceImplementation)
             {
                 if ((mods & DeclarationModifiers.Abstract) != 0)
@@ -258,11 +274,11 @@ internal static DeclarationModifiers AdjustModifiersForAnInterfaceMember(Declara
             {
                 mods &= ~DeclarationModifiers.Sealed;
             }
-            else if ((mods & (DeclarationModifiers.Private | DeclarationModifiers.Partial | DeclarationModifiers.Virtual | DeclarationModifiers.Abstract)) == 0)
+            else if ((mods & (DeclarationModifiers.Private | DeclarationModifiers.Virtual | DeclarationModifiers.Abstract)) == 0 && notPartialOrNewPartialBehavior)
             {
                 Debug.Assert(!isExplicitInterfaceImplementation);
 
-                if (hasBody || (mods & (DeclarationModifiers.Extern | DeclarationModifiers.Sealed)) != 0)
+                if (hasBody || (mods & (DeclarationModifiers.Extern | DeclarationModifiers.Partial | DeclarationModifiers.Sealed)) != 0)
                 {
                     if ((mods & DeclarationModifiers.Sealed) == 0)
                     {
@@ -279,18 +295,6 @@ internal static DeclarationModifiers AdjustModifiersForAnInterfaceMember(Declara
                 }
             }
 
-            if ((mods & DeclarationModifiers.AccessibilityMask) == 0)
-            {
-                if ((mods & DeclarationModifiers.Partial) == 0 && !isExplicitInterfaceImplementation)
-                {
-                    mods |= DeclarationModifiers.Public;
-                }
-                else
-                {
-                    mods |= DeclarationModifiers.Private;
-                }
-            }
-
             return mods;
         }
 

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

@@ -569,7 +569,7 @@ private DeclarationModifiers MakeModifiers(SyntaxTokenList modifiers, bool expli
             // Proper errors must have been reported by now.
             if (isInterface)
             {
-                mods = ModifierUtils.AdjustModifiersForAnInterfaceMember(mods, !isFieldLike, explicitInterfaceImplementation);
+                mods = ModifierUtils.AdjustModifiersForAnInterfaceMember(mods, !isFieldLike, explicitInterfaceImplementation, forMethod: false);
             }
 
             return mods;

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

@@ -797,7 +797,8 @@ private static DeclarationModifiers AddImpliedModifiers(DeclarationModifiers mod
             if (containingTypeIsInterface)
             {
                 mods = ModifierUtils.AdjustModifiersForAnInterfaceMember(mods, hasBody,
-                                                                         methodKind == MethodKind.ExplicitInterfaceImplementation);
+                                                                         methodKind == MethodKind.ExplicitInterfaceImplementation,
+                                                                         forMethod: true);
             }
             else if (methodKind == MethodKind.ExplicitInterfaceImplementation)
             {

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

@@ -482,7 +482,7 @@ private static (DeclarationModifiers modifiers, bool hasExplicitAccessMod) MakeM
             // Proper errors must have been reported by now.
             if (isInterface)
             {
-                mods = ModifierUtils.AdjustModifiersForAnInterfaceMember(mods, accessorsHaveImplementation, isExplicitInterfaceImplementation);
+                mods = ModifierUtils.AdjustModifiersForAnInterfaceMember(mods, accessorsHaveImplementation, isExplicitInterfaceImplementation, forMethod: false);
             }
 
             if (isIndexer)