Hmm, maybe that's a complex solution for a situation that'll be less and less common, especially when `map' & co. are rewritten in Scheme.

WDYT?

This is with `map' implemented in C:

#v+
scheme@(guile-user)> (map (lambda (x) (values x x)) (iota 2))
$5 = (#<values (0 0)> #<values (1 1)>)
scheme@(guile-user)> ,x map
Disassembly of #<procedure map ( . _)>:

   0    (assert-nargs-ge 0 2)          
   3    (push-rest 0 2)                
   6    (object-ref 0)                  ;; #<pointer 0x7f4ee0395d20>
   8    (subr-call 3)                  
#v-

Instead, the result should be '(0 1).

Conversely:

#v+
scheme@(guile-user)> (map (lambda (x) (identity (values x x))) (iota 2))
$6 = (0 1)
scheme@(guile-user)> ,x identity
Disassembly of #<procedure identity (x)>:

   0    (assert-nargs-ee/locals 1)     
   2    (local-ref 0)                   ;; `x'
   4    (return)                       
#v-