Merge pull request #1995 from ruby/update-docs

Update `syntax.md`

Author: soutaro

docs/syntax.md

@@ -750,16 +750,15 @@ $LOAD_PATH: Array[String]
 ### Generics
 
 ```markdown
-_module-type-parameter_ ::= _generics-unchecked_ _generics-variance_ _type-variable_ _generics-bound_
+_module-type-parameter_ ::= _generics-unchecked_ _generics-variance_ _type-variable_ _generics-bound_ _default-type_
 
 _method-type-param_ ::= _type-variable_ _generics-bound_
 
-_generics-bound_ ::=                       (No type bound)
-                   | `<` _bound-type_      (The generics parameter is bounded)
+_generics-bound_ ::=                 (No type bound)
+                   | `<` _type_      (The generics parameter is bounded)
 
-_bound-type_ ::= _class-name_ _type-arguments_       (Class instance type)
-               | _interface-name_ _type-arguments_   (Interface type)
-               | `singleton(` _class-name_ `)`       (Class singleton type)
+_default-type_ ::=                    (No default type)
+                 | `=` _type_         (The generics parameter has default type)
 
 _generics-variance_ ::=               (Invariant)
                       | `out`         (Covariant)
@@ -826,14 +825,27 @@ class PrettyPrint[T < _Output]
 end
 ```
 
-If a type parameter has an upper bound, the type parameter must be instantiated with types that is a subclass of the upper bound.
+If a type parameter has an upper bound, the type parameter must be instantiated with types that is a subtype of the upper bound.
 
 ```rbs
 type str_printer = PrettyPrint[String]    # OK
 type int_printer = PrettyPrint[Integer]   # Type error
 ```
 
-The upper bound must be one of a class instance type, interface type, or class singleton type.
+The generics type parameter of modules, classes, interfaces, or type aliases can have a default type.
+
+```rbs
+interface _Foo[T = untyped]
+end
+
+interface _Bar[T, S = untyped]
+end
+
+type foo = _Foo          # equivalent to _Foo[untyped]
+type bar = _Bar[String]  # equivalent to _Bar[String, untyped]
+```
+
+Type parameters with default types cannot appear before type parameters without default types. The generic method type parameters cannot have the default types.
 
 ### Directives