Fix handling of COM interfaces with same method name

crates/libs/interface/src/lib.rs

@@ -237,8 +237,11 @@ impl Interface {
                     quote! {
                         unsafe extern "system" fn #name<Identity: ::windows_core::IUnknownImpl<Impl = Impl>, Impl: #trait_name, const OFFSET: isize>(this: *mut ::core::ffi::c_void, #(#args),*) #ret {
                             let this = (this as *const *const ()).offset(OFFSET) as *const Identity;
-                            let this = (*this).get_impl();
-                            this.#name(#(#params),*).into()
+                            let this_impl: &Impl = (*this).get_impl();
+                            // We use explicit <Impl as IFoo_Impl> so that we can select the correct method
+                            // for situations where IFoo3 derives from IFoo2 and both declare a method with
+                            // the same name.
+                            <Impl as #trait_name>::#name(this_impl, #(#params),*).into()
                         }
                     }
                 } else {

crates/tests/implement_core/src/com_object.rs

@@ -12,16 +12,25 @@ unsafe trait IFoo: IUnknown {
     fn get_x(&self) -> u32;
 
     fn get_self_as_bar(&self) -> IBar;
+
+    fn common(&self) -> u32;
 }
 
 #[interface("687eb4b2-6df6-41a3-86c7-4b04b94ad2d8")]
 unsafe trait IBar: IUnknown {
     fn say_hello(&self);
+
+    fn common(&self) -> u64;
+}
+
+#[interface("4351c285-97ad-450a-b445-8795632d2fb9")]
+unsafe trait IBar2: IBar {
+    fn common(&self) -> f32;
 }
 
 const APP_SIGNATURE: [u8; 8] = *b"cafef00d";
 
-#[implement(IFoo, IBar)]
+#[implement(IFoo, IBar, IBar2)]
 struct MyApp {
     // We use signature to verify field offsets for dynamic casts
     signature: [u8; 8],
@@ -38,12 +47,26 @@ impl IFoo_Impl for MyApp {
         let outer = MyApp_Impl::from_inner_ref(self);
         outer.to_interface()
     }
+
+    unsafe fn common(&self) -> u32 {
+        100
+    }
 }
 
 impl IBar_Impl for MyApp {
     unsafe fn say_hello(&self) {
         println!("Hello!");
     }
+
+    unsafe fn common(&self) -> u64 {
+        1_000_000_000_000
+    }
+}
+
+impl IBar2_Impl for MyApp {
+    unsafe fn common(&self) -> f32 {
+        std::f32::consts::PI
+    }
 }
 
 impl Borrow<u32> for MyApp {
@@ -376,6 +399,23 @@ fn dynamic_cast() {
     assert_eq!(dyn_app_owned_2.signature, APP_SIGNATURE);
 }
 
+// Test that we can invoke the correct method in situations where two different
+// interfaces declare a method with the same name, including situations where
+// one of the interfaces inherits from the other.
+#[test]
+fn common_method_name() {
+    let app = MyApp::new(42);
+
+    let ifoo: IFoo = app.to_interface();
+    assert_eq!(unsafe { ifoo.common() }, 100);
+
+    let ibar: IBar = app.to_interface();
+    assert_eq!(unsafe { ibar.common() }, 1_000_000_000_000);
+
+    let ibar2: IBar2 = app.to_interface();
+    assert_eq!(unsafe { ibar2.common() }, std::f32::consts::PI);
+}
+
 // This tests that we can place a type that is not Send in a ComObject.
 // Compilation is sufficient to test.
 #[implement(IBar)]
@@ -387,6 +427,10 @@ impl IBar_Impl for UnsendableThing {
     unsafe fn say_hello(&self) {
         println!("{}", self.cell.get());
     }
+
+    unsafe fn common(&self) -> u64 {
+        0
+    }
 }
 
 static_assertions::assert_not_impl_all!(UnsendableThing: Send, Sync);
@@ -401,6 +445,10 @@ impl IBar_Impl for SendableThing {
     unsafe fn say_hello(&self) {
         println!("{}", *self.arc);
     }
+
+    unsafe fn common(&self) -> u64 {
+        0
+    }
 }
 
 static_assertions::assert_impl_all!(SendableThing: Send, Sync);