design: type parameters: promote embedded methods from constraint

Change-Id: If4be0f74932c1ec504833549fb01a047dcf8335f
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/251444
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/design/go2draft-type-parameters.md b/design/go2draft-type-parameters.md
index 3dcb07b..54b1604 100644
--- a/design/go2draft-type-parameters.md
+++ b/design/go2draft-type-parameters.md
@@ -3710,6 +3710,27 @@ func (l *Lockable[T]) Set(v T) {
 }
 ```
 
+### Embedded type parameter methods
+
+When a generic type is a struct, and the type parameter is embedded as
+a field in the struct, any methods of the type parameter's constraint
+are promoted to be methods of the struct.
+
+```Go
+// NamedInt is an int with a name. The name can be any type with
+// a String method.
+type NamedInt[Name fmt.Stringer] struct {
+	Name
+	val int
+}
+
+// Name returns the name of a NamedInt.
+func (ni NamedInt[Name]) Name() string {
+	// The String method is promoted from the embedded Name.
+	return ni.String()
+}
+```
+
 ### Embedded instantiated type
 
 When embedding an instantiated type, the name of the field is the name