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    <p>Here's how I understand this passage. At least it makes sense to me, so I may claim understanding it :)</p> <p>I think that what it is trying to say is that the <code>(values ...)</code> form must be compatible with the arguments lambda list of the function, which, if called from <code>multiple-value-call</code> would have received these arguments correctly.</p> <p>In other words, suppose there are two functions: <code>f</code> and <code>g</code>. <code>f</code> returns values: <code>(integer &amp;rest more-integers)</code> - in human language: it can return one ore more integers. Then, if you want to call <code>(multiple-value-call #'g (f))</code> you must (you is a compiler, actually) make sure that the signature of <code>g</code> is <code>(ftype (function (integer &amp;rest more-integers) t))</code>. The return type of <code>g</code> doesn't matter.</p> <p>Here's my attempt at making an example illustrating it:</p> <pre><code>CL-USER&gt; (declaim (ftype (function () (values integer &amp;rest more-integers)) f)) ; No value CL-USER&gt; (defun f () (values 1 2)) F CL-USER&gt; (declaim (ftype (function (integer &amp;rest more-integers) t) g)) ; No value CL-USER&gt; (defun g (x &amp;rest rest) (reduce #'+ rest :initial-value x)) G CL-USER&gt; (multiple-value-call #'g (f)) 3 CL-USER&gt; (declaim (ftype (function (single-float) t) x)) ; No value CL-USER&gt; (defun x (y) y) X CL-USER&gt; (multiple-value-call #'g (x 1.5)) ; Evaluation aborted on #&lt;TYPE-ERROR expected-type: INTEGER datum: 1.5&gt;. </code></pre>
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    1. POWhat do &OPTIONAL and &REST mean in a VALUES type specifier?
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    1. COYour answer seems identical to the one given by the SBCL dev in the original question. However, I like yours better because in addition to describing what &OPTIONAL and &REST mean in this context, it actually makes sense of what the spec tries to say.
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    2. COHowever, your example is not apropos. In the last line, the error is in the type passed to X, not in the type returned by it. Try this: (declaim (ftype (function (integer number) t) g)) (defun g (x y) (+ x y)) (declaim (ftype (function () (values integer &optional)) x)) (defun x () 230) (multiple-value-call #'g (x)) ; Evaluation aborted on #<SB-INT:SIMPLE-PROGRAM-ERROR "invalid number of arguments: ~S" {100408A463}>.
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    3. CONOTE: The type correspondence between the function's value-type (return type) and another's parameter list is only checked if &OPTIONAL or &REST are present in the VALUES type specifier for the former's value-type declaration.
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