Add disjoint requirement for dependent type inference

Fixes #386.

lang/spec.html

@@ -2007,7 +2007,11 @@ <h4 id="functions">Functions</h4>
 type-reference occurring in a return-type-descriptor can refer to a parameter
 name if the type of the parameter is a subtype of <code>typedesc</code>. If p is
 such a parameter, then a reference to p in the return-type-descriptor denotes
-the type that is the value of p. The return-type-descriptor thus denotes a
+the type that is the value of p. If the default value for p is
+<code>&lt;&gt;</code> and p occurs in the return-type-descriptor in a union p|T,
+then for every basic type B, either the intersection of B and T must be empty,
+or the intersection of typedesc&lt;B&gt; and the declared type of p must be
+empty, or both. The return-type-descriptor thus denotes a
 distinct set of shapes for each invocation of the function. A function-signature
 with a return-type-descriptor that uses a type-reference to refer to a parameter
 name in this way is said to be <em>dependently-typed</em>. Functions with