bug#39277:

[Hadrien Lacour]

Sorry for not really contributing, I must provid the excuse that I don't have
enough time or energy right now.

About Tcl syntax rules, Tcl(3tcl) (or Tcl(n) on gentoo) explains it simply:
...
[4] Double quotes.
        If the first character of a word is double-quote (“"”) then the word is
        terminated by the next double-quote character.  If semi-colons, close
        brackets, or white space characters (including newlines) appear between 
the
        quotes then they are treated as ordinary characters and included in the
        word.  Command substitution, variable substitution, and backslash
        substitution are performed on the characters between the quotes as
        described below. The double-quotes are not retained as part of the word.
...
[6] Braces.
        If the first character of a word is an open brace (“{”) and rule [5] 
does
        not apply, then the word is terminated by the matching close brace 
(“}”).
        Braces nest within the word: for each additional open brace there must 
be
        an additional close brace (however, if an open brace or close brace 
within
        the word is quoted with a backslash then it is not counted in locating 
the
        matching close brace).  No substitutions are performed on the characters
        between the braces except for backslash-newline substitutions described
        below, nor do semi-colons, newlines, close brackets, or white space 
receive
        any special interpretation.  The word will consist of exactly the
        characters between the outer braces, not including the braces 
themselves.
...

To put simply, braces act like sh's single quotes and double quotes are
basically the same (only $ or [] is substituted).