Qt does not seem to request the right font properties to fontconfig

If I have a font file that only offers a regular version of a font, and inside an application I set that font in bold, Qt doesn't request the bold version to fontconfig but only the regular one.

This can be seen by exporting FC_DEBUG=4 (or 1, or 2) and checking the handling of the request by fontconfig. The initial request (for a bold font in the application) is just:

FcConfigSubstitute Pattern has 6 elts (size 16)
        family: "Open Sans"(s)
        pixelsize: 19(f)(s)
        file: "/home/peppe/.fonts/OpenSans-Regular.ttf"(s)
        index: 0(i)(s)
        lang: "it"(w)
        prgname: "test"(s)

This blocks a user from having fontconfig rules that for instance may provide another font family / file in case someone asks for Open Sans in bold.

On the other hand, if a bold font file is then installed, the request then points directly to that bold file:

FcConfigSubstitute Pattern has 6 elts (size 16)
        family: "Open Sans"(s)
        pixelsize: 19(f)(s)
        file: "/home/peppe/.fonts/OpenSans/OpenSans-Bold.ttf"(s)
        index: 0(i)(s)
        lang: "it"(w)
        prgname: "test"(s)

But once more there's no weight specified here.