bug#14595: Erroneous composition of lambda in emacs-lisp buffers with prog-prettify-symbols enabled

[Juanma Barranquero <lekktu@gmail.com>]

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Package: emacs
Version: 24.3.50


With a test file lambda.el which contains 1000 identical lines:

(lambda (x) x)

emacs -Q --eval "(setq-default prog-prettify-symbols t)" lambda.el

In most cases, lambda is correctly composed to λ; in a few, it is
composed (apparently) like a supperposition of the characters in
"lambda" (attached image included).

If I visit lambda.el as in the example above, and scroll down the
buffer, I get 26 such failures, always at the same lines (168, 280,
364, 432, 489, 538, 581, 619, 654, 686, 715, 742, 767, 791, 813, 834,
854, 873, 891, 908, 924, 940, 955, 970, 984 and 998); however, if upon
visiting the file I go to the end of the buffer and scroll up, I get
many more failures (about 60) at diferent lines, but also consistently
the same lines.


describe-char for a correct composition:

             position: 2492 of 15000 (17%), column: 1
            character: l (displayed as l) (codepoint 108, #o154, #x6c)
    preferred charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x6C
               script: latin
               syntax: w which means: word
             category: .:Base, L:Left-to-right (strong), a:ASCII,
l:Latin, r:Roman
             to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
          buffer code: #x6C
            file code: #x6C (encoded by coding system utf-8-dos)
              display: composed to form "lambda" (see below)

Composed with the following character(s) "ambda" by the rule:
(?λ)
The component character(s) are displayed by these fonts (glyph codes):
 λ: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso10646-1 (#x21C)
See the variable `reference-point-alist' for the meaning of the rule.

Character code properties: customize what to show
  name: LATIN SMALL LETTER L
  general-category: Ll (Letter, Lowercase)
  decomposition: (108) ('l')

There are text properties here:
  composition          [Show]
  face                 font-lock-keyword-face
  fontified            t


describe-char for a wrong one:

             position: 2507 of 15000 (17%), column: 1
            character: l (displayed as l) (codepoint 108, #o154, #x6c)
    preferred charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x6C
               script: latin
               syntax: w which means: word
             category: .:Base, L:Left-to-right (strong), a:ASCII,
l:Latin, r:Roman
             to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
          buffer code: #x6C
            file code: #x6C (encoded by coding system utf-8-dos)
              display: composed to form "lambda" (see below)

Composed with the following character(s) "ambda" by the rule:
(?l ?a ?m ?b ?d ?a)
The component character(s) are displayed by these fonts (glyph codes):
 l: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x4F)
 a: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x44)
 m: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x50)
 b: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x45)
 d: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x47)
 a: uniscribe:-outline-Courier
New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x44)
See the variable `reference-point-alist' for the meaning of the rule.

Character code properties: customize what to show
  name: LATIN SMALL LETTER L
  general-category: Ll (Letter, Lowercase)
  decomposition: (108) ('l')

There are text properties here:
  composition          [Show]
  face                 font-lock-keyword-face
  fontified            t

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