Require `override` keyword for overriding default interface methods. (#7458)

* Require `override` keyword for overriding default interface methods.

* Update doc.

* Fix test.

1 files changed, 11 insertions, 0 deletions

diff --git a/docs/user-guide/06-interfaces-generics.md b/docs/user-guide/06-interfaces-generics.md
index 1912fcb02..bb19fd776 100644
--- a/docs/user-guide/06-interfaces-generics.md
+++ b/docs/user-guide/06-interfaces-generics.md
@@ -58,6 +58,17 @@ interface IFoo
 struct MyType : IFoo {}
 ```
 
+A concrete type that provides its overriding implementation to an interface method requirement that has a default implementation must be explicitly marked as 'override'. For example:
+
+```slang
+struct MyType2 : IFoo
+{
+    // Explicitly mark `getVal` as `override` is needed
+    // because `IFoo.getVal` has a body.
+    override int getVal() { return 1; }
+}
+```
+
 Generics
 ---------------------