[External] [stdlib] Add Defaultable trait (#39797)

[External] [stdlib] Add `Defaultable` trait

Add a `Defaultable` trait for describing a type which has a constructor
with no arguments.

Co-authored-by: Helehex <[email protected]>
Closes modularml#2526
MODULAR_ORIG_COMMIT_REV_ID: aed69a25ecf531873f944466a1cb5d7652dd7425

Signed-off-by: Lukas Hermann <[email protected]>

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.