Logical block prefix non-constantness false-positive in FORMAT.

[Samium Gromoff]

deepfire@auriga:~$ sbcl --no-userinit --eval '(format nil "~@<~@< ~;~A~:@>~:@>" nil)'
This is SBCL 1.0.33.2, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses. See the CREDITS and COPYING files in the
distribution for more information.

debugger invoked on a SB-FORMAT:FORMAT-ERROR in thread #<THREAD "initial thread" RUNNING {1002A6E041}>:
  error in FORMAT: cannot include format directives inside the prefix segment of ~<...~:>
  ~@<~@< ~;~A~:@>~:@>
        ^
See also:
  The ANSI Standard, Section 22.3.5.2

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE] Ignore runtime option --eval "(format nil \"~@<~@< ~;~A~:@>~:@>\" nil)".
  1: [ABORT ] Skip rest of --eval and --load options.
  2: Skip to toplevel READ/EVAL/PRINT loop.
  3: [QUIT ] Quit SBCL (calling #'QUIT, killing the process).

((FLET SB-FORMAT::EXTRACT-STRING) (" " #<>) T)
0] * deepfire@auriga:~$ sbcl --no-userinit --eval '(format nil "~@<~@<~;~A~:@>~:@>" nil)'
This is SBCL 1.0.33.2, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses. See the CREDITS and COPYING files in the
distribution for more information.
*

[Nikodemus Siivola (nikodemus)]

I'm slow, so please clarify: the second case should complain as well, instead of printing "NIL"?

[Nikodemus Siivola (nikodemus)]

For those at the back of the class:

(format nil "~@<~A~@<~;~A~:@>~:@>" "<outer-block-prefix>" "<inner-block-body>")

 => "<outer-block-prefix><inner-block-body>"
 SHOULD be an error

...add a space at the start of the inner block prefix, and:

(format nil "~@<~A~@< ~;~A~:@>~:@>" "<outer-block-prefix>" "<inner-block-body>")

=| ERROR, which is right