C/C++ Live Template "Edit Variables" dialog doesn't show an editable Expression field (unlike other languages)

Hi,

I'm trying to create a custom Live Template in CLion for C/C++ that generates an adaptive comment separator line, where the number of dashes is computed dynamically from a groovyScript() macro depending on the current line's indentation and the length of a typed variable.

The template body is:

// $LEFT$ $TEXT$ $RIGHT$

expected result : a line of 80 chars with the number of dash computed automagically. 

// ----------- the text -----------

LEFT and RIGHT are meant to hold an expression like:

groovyScript("def d=_editor.getDocument(); def l=_editor.getCaretModel().getLogicalPosition().line; def s=d.getLineStartOffset(l); def e=d.getLineEndOffset(l); def t=d.getText(new com.intellij.openapi.util.TextRange(s,e)); def i=0; while(i<t.length()&&Character.isWhitespace(t.charAt(i))) i++; def tl=_1==null?0:_1.length(); def budget=80-i-3-2-tl; def n=(int)(budget/2); if(n<1) n=1; return '-'*n", TEXT)

The problem: when the template's applicable context is set to C/C++, the "Edit Variables" dialog does not give me any editable field to type this expression into — I only get a list of predefined macros to pick from, with no way to enter free text/a custom expression.

When I set the applicable context to another language instead, the Expression field behaves as expected (a normal editable text field with autocomplete, where I can type a custom groovyScript(...) expression).

Is this a known limitation of Live Templates scoped to C/C++, or a bug? Is there a workaround to define a custom expression-based variable for a C/C++-scoped Live Template?

For context: I asked an AI coding assistant for help configuring this, and it proposed the setup above, but we couldn't get the Expression field to become editable in the C/C++ context specifically — everything works as described for other languages.

CLion version: [precise ta version, ex. 2026.2.1]
OS: [fedora 43]

Thanks!