Does the `-i` / `--interpolate` flag produce nothing if a font has only a single master and instance?

I’m finding that, if I add the `-i` / `--interpolate` flag to a fontmake run, when trying to build from a simple Glyphs file that has just a single master and instance, I get no output.

On one hand, that would make some sense... if there’s nothing to interpolate, why would fontmake output anything?

On the other hand, it becomes an issue, because it seems that the gftools builder will add `-i` to most fontmake calls, even when not needed. (See https://github.com/googlefonts/gftools/issues/878)

Additionally, if fontmake decides to not output anything due to a flag that is deemed irrelevant, I would expect to receive an error for that flag.

Reproduction:
https://github.com/arrowtype/fontmake-issue-1080-reproduction

If I’m just overlooking something obvious, I would also appreciate learning what I’m missing!

Thanks so much for any help, as always.

I’m using fontmake `3.8.1`.

Start by reproducing the fontmake 3.8.1 command with -i/--interpolate using the linked minimal Glyphs file, then trace how that CLI flag is handled for a single master and instance. Compare the result with the expected behavior implied by the report: produce output or return an explicit error when interpolation is inapplicable.

Written by the indexing model from the issue text.