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<h4 class="subsection">4.2.4 Implementation Limitations</h4>
<p>Ideally, the compiler would consider <em>all</em> type declarations to
be assertions, so that adding type declarations to a program, no
matter how incorrect they might be, would <em>never</em> cause undefined
behavior. However, the compiler is known to fall short of this goal in
two areas:
<ul>
<li><em>Proclaimed</em> constraints on argument and result types of a
function are supposed to be checked by the function. If the function
type is proclaimed before function definition, type checks are
inserted by the compiler, but the standard allows the reversed order,
in which case the compiler will trust the declaration.
<li>The compiler cannot check types of an unknown number of values; if the
number of generated values is unknown, but the number of consumed is
known, only consumed values are checked.
<p>For example,
<pre class="lisp"> (defun foo (x)
(the integer (bar x)))
</pre>
<p>causes the following compiler diagnostic to be emitted:
<pre class="example"> ; note: type assertion too complex to check:
; (VALUES INTEGER &REST T).
</pre>
<p>A partial workaround is instead write:
<pre class="lisp"> (defun foo (x)
(the (values integer &optional) (bar x)))
</pre>
</ul>
<p>These are important issues, but are not necessarily easy to fix, so
they may, alas, remain in the system for a while.
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