Type mismatch in the expansion of (setf schar)

[Timofei Shatrov]

Minimal example:

(defun weird-warning (str)
  (let ((copy (copy-seq str)))
    (setf (char copy 0) #\HIRAGANA_LETTER_SMALL_TU)))

Upon compiling the following warning appears:

  warning:
    Derived type of SB-C::NEW-VALUE is
      (VALUES EXTENDED-CHAR &OPTIONAL),
    conflicting with its asserted type
      BASE-CHAR.

This does not happen in SBCL 1.3.3 but does happen in SBCL 1.3.8 and later.

The code works, but the warning trips SLIME per-function compilation and asdf:load-system.

[Stas Boukarev (stassats)]

This is actually
(defun foo (a)
  (setf (schar a 0) #\HIRAGANA_LETTER_SMALL_TU))

[Stas Boukarev (stassats)]

In 1ac88e72db13e6586b8ceebff468b1fa108178cb