[stdlib] add Defaultable trait

stdlib/src/builtin/value.mojo

@@ -99,6 +99,40 @@ trait Copyable:
  ...
 
 
+trait Defaultable:
+ """The `Defaultable` trait describes a type with a default constructor.
+
+ Implementing the `Defaultable` trait requires the type to define
+ an `__init__` method with no arguments:
+
+ ```mojo
+ struct Foo(Defaultable):
+ var s: String
+
+ fn __init__(inout self):
+ self.s = "default"
+ ```
+
+ You can now construct a generic `Defaultable` type:
+
+ ```mojo
+ fn default_init[T: Defaultable]() -> T:
+ return T()
+
+ var foo = default_init[Foo]()
+ print(foo.s)
+ ```
+
+ ```plaintext
+ default
+ ```
+ """
+
+ fn __init__(inout self):
+ """Create a default instance of the value."""
+ ...
+
+
 trait CollectionElement(Copyable, Movable):
  """The CollectionElement trait denotes a trait composition
  of the `Copyable` and `Movable` traits.