Hi,

The first thing that came to my mind reading your proposal was "what if
the expression between braces contains a legit single colon? Is there a
subset of valid expressions that can be used in a replacement field?"

You address the first point in the section Error Cases in a way that is
unsatisfying in my opinion. Quote:

    For example the first line of the following has a single semi-colon
    : between 2 and 6, which will be treated as separating an optional
    format-spec, and will thus convert but fail to compile later:

    F"{check() ? 2 : 6}"; // ERROR: converts, but fails to compile later
    F"{check() ? 2 : 6:}"; // ok

I like the simplicity of your approach but IMHO we cannot have such
approximate subset of expressions for replacement-fields. This should be
either any expression (good luck parsing that ;)) or else something like
a single identifier (but then frustration will arise).

Julien