make it explicit that a license exception, alone, cannot be a valid license expression

[alpianon]

see fsfe/reuse-tool#890 and and spdx/Spdx-Java-Library#227

TL;DR: Qt-GPL-exception-1.0 (or any other "standalone" license exception) is not a valid license expression that can be put in LicenseInfoInFile.

That is implied by what is written in Annex D.1, but it would be better to make it more explicit, to avoid issues like the ones mentioned above

[pmonks]

The ABNF grammar in Annex D.1 makes this explicit, and I've always understood the sentence "The exact syntax of license expressions is described below in ABNF." to mean that that grammar is normative.

[alpianon]

The ABNF grammar in Annex D.1 makes this explicit, and I've always understood the sentence "The exact syntax of license expressions is described below in ABNF." to mean that that grammar is normative.

What about adding something like "For the sake of clarity, a license exception in isolation is not a valid license expression"?

I'm pretty sure there are other tools out there (eg. Fossology) that incur in the same mistake, so make it clearer would do no harm and may help avoiding such mistake

[pmonks]

It seems to me that the logical conclusion of that argument would be that the entire ABNF grammar needs to be duplicated and translated to plain English, which I'd argue is inappropriate in a technical specification like the SPDX spec. ABNF is a good choice for describing this kind of thing precisely and succinctly.

[goneall]

Agree with @pmonks - ABNF should be the "source of truth" for the license expression.

@kestewart - thoughts?