bug#39277:

[Hadrien Lacour <hadrien.lacour@posteo.net>]

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).