DestClause is intra-function only.

Now I explicitly stated that the destination clause is used only for
designating intra-function destinations, that is, cannot have basic
blocks in other function versions as destinations. This does not change
the semantics, but just make it explicit.

instruction-set.rst

@@ -201,8 +201,11 @@ argList
     *list of SSA variable*: Arguments to the destination basic block
 
 The destination clause designates a basic block as the destination of branching,
-either normal or exceptional.  It takes many arguments, which will be received
-by the normal parameters of ``dest``.  The arguments must match the number and
+either normal or exceptional. The destination must be in the same function
+version as the instruction that includes the destination clause.
+
+A destination clause takes many arguments, which will be received by the normal
+parameters of the ``dest`` basic block.  The arguments must match the number and
 the types of the normal parameters. The exceptional parameter does not need to
 be explicitly passed to.