bug#48477: 28.0.50; Seemingly incorrect codegen with multiple string-matching pcase patterns

[Mattias Engdegård]

Serves me right for trying to be clever! Very sorry about that.

Matches would always succeed because the outcome was erroneously transformed 
into a match against a plain pcase variable which never fails. For example, the 
pattern

 (rx (let x "a"))

would expand to
    
 (and (pred stringp)
      (app (lambda (s) (and (string-match (rx (group-n 1 "a")) s)
                            (match-string 1 s)))
           x))

which cannot fail (as long as the input is a string).  Patterns with two or 
more named submatches are not affected because of the structural match used, 
and zero submatches were treated specially anyway.

Please try the attached patch. It encodes non-matches as the number 0 (any 
non-nil non-string value would have done; 0 is cheap to create and test). The 
above pattern now expands to

 (and (pred stringp)
      (app (lambda (s) (if (string-match (rx (group-n 1 "a")) s)
                           (match-string 1 s)
                         0))
           (and x (pred (not numberp)))))

0001-Fix-pcase-rx-patterns-with-a-single-named-submatch-b.patch
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