[bug #55154] .tr has undocumented and inconsistent space-character restrictions

[Dave]

Follow-up Comment #13, bug #55154 (group groff):

[comment #11 comment #11:]
> I presume this is due to this explanation in the Texinfo manual:
> 
>  -- Request: .char c ['"'][contents]
>      Every time C is to be output, CONTENTS is processed in a
> temporary environment and the result encapsulated in a node.
> 
> The temporary environment being unaware of the rest of the line,
> it can only turn \~ into a node that is the width of an ordinary
> unbreakable space.

The below (requiring a recent groff build, as it uses .pline) is consistent
with this hypothesis (though not proof of it, as the precise content of the
two composite_node nodes is unknown).

$ cat 55154.tr
.char b \~
abc cba
.pline
.brp
.tm
a\~c c\~a
.pline
.brp
$ groff 55154.tr > /dev/null
{type: line_start_node, diversion level: 0},
{type: glyph_node, character: "a", diversion level: 0},
{type: composite_node, diversion level: 0},
{type: glyph_node, character: "c", diversion level: 0},
{type: word_space_node, diversion level: 0},
{type: glyph_node, character: "c", diversion level: 0},
{type: composite_node, diversion level: 0},
{type: glyph_node, character: "a", diversion level: 0}

{type: line_start_node, diversion level: 0},
{type: glyph_node, character: "a", diversion level: 0},
{type: unbreakable_space_node, diversion level: 0},
{type: glyph_node, character: "c", diversion level: 0},
{type: word_space_node, diversion level: 0},
{type: glyph_node, character: "c", diversion level: 0},
{type: unbreakable_space_node, diversion level: 0},
{type: glyph_node, character: "a", diversion level: 0}




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