new, stricter interface rule:

in x.(T) x cannot be a nil (uninitialized) interface variable.

remove TODO per discussion.

DELTA=3  (1 added, 1 deleted, 1 changed)
OCL=29123
CL=29134

doc/go_spec.html

@@ -2210,7 +2210,8 @@ x.(T)
 </pre>
 
 <p>
-asserts that the value stored in <code>x</code> is of type <code>T</code>.
+asserts that <code>x</code> is not the zero interface value
+and that the value stored in <code>x</code> is of type <code>T</code>.
 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
 The type of <code>x</code> must be an interface type.
 </p>
@@ -2220,7 +2221,6 @@ that the dynamic type of <code>x</code> is identical to the type <code>T</code>
 (§Type identity and compatibility).
 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
 of <code>T</code> implements the interface <code>T</code> (§Interface types).
-<font color=red>TODO: gri wants an error if x is already of type T.</font>
 </p>
 <p>
 If the type assertion holds, the value of the expression is the value