Add nullable types

Fixed #196.

Author: nikic

spec/13-functions.md

@@ -77,6 +77,9 @@ return-type:
  ':' 'void'
 
 type-declaration:
+ '?'? base-type-declaration
+
+base-type-declaration:
  'array'
  'callable'
  'iterable'
@@ -123,6 +126,9 @@ default-argument-specifier:
  : void
 
 <i id="grammar-type-declaration">type-declaration:</i>
+ ?<sub>opt</sub> <i><a href="#grammar-base-type-declaration">base-type-declaration</a></i>
+
+<i id="grammar-base-type-declaration">base-type-declaration:</i>
  array
  callable
  iterable
@@ -218,7 +224,9 @@ Parameters typed with *scalar-type* are accepted if they pass the type check for
 as [described below](#type-check-modes). Once the checks have been passed, the parameter types are always of the scalar type
 specified (or `NULL` if `NULL` is allowed).
 
-If a parameter has a type declaration, `NULL` is not accepted unless it has a default value that evaluates to `NULL`.
+If a parameter has a type declaration, `NULL` is not accepted unless the
+type is nullable. A type is nullable if it is prefixed with `?` or if the
+parameter has a default value that evaluates to `NULL`.
 
 The default value for a typed parameter must be of the type specified, or `NULL`,
 and conversion is not be performed for defaults, regardless of the mode.

spec/19-grammar.md

@@ -929,6 +929,9 @@ The grammar notation is described in [Grammars section](09-lexical-structure.md#
  : void
 
 <i id="grammar-type-declaration">type-declaration:</i>
+ ?<sub>opt</sub> <i><a href="#grammar-base-type-declaration">base-type-declaration</a></i>
+
+<i id="grammar-base-type-declaration">base-type-declaration:</i>
  array
  callable
  iterable