I wonder if it would make sense to have an NodeWarningType::Info message when there are no Bezier splines in the curve. Maybe it would help someone who expected it to work on poly splines or something.
NodeGeometrySetCurveHandles would probably make sense, since we'll have other nodes that deal with handle types.

source/blender/makesrna/intern/rna_nodetree.c
9447–9466	I really like your descriptions here! Two comments: For better or worse, the periods are added automatically by the tooltip system. We have icons for these, which we might as well use-- `ICON_HANDLE_VECTOR`, etc.
9470–9471	No periods in the descriptions Handle should probably be plural
source/blender/nodes/geometry/nodes/node_geo_curve_handles.cc
16–21 ↗	(On Diff #39788)	Generally the different modules / prefixes get newlines between them.
55 ↗	(On Diff #39788)	Thsee parentheses are unnecessary: `(params.node().storage)` -> `params.node().storage`
68 ↗	(On Diff #39788)	`handlesd` -> `handles`
81–94 ↗	(On Diff #39788)	Best to just do this once, rather than in the loop. Also, you can make it a separate function, then you can use `return` in the switch cases, which is a bit nicer.

This revision now requires changes to proceed.Jul 22 2021, 4:56 AM

Code cleanup and Hans' notes applied

This revision was not accepted when it landed; it landed in state Needs Review.Jul 22 2021, 4:56 PM

This revision was automatically updated to reflect the committed changes.

I'll make a couple small changes when committing.

source/blender/nodes/geometry/nodes/node_geo_curve_set_handles.cc
69	This function doesn't have a return for all possible values of type: BLI_assert_unreachable(); return BezierSpline::HandleType::Auto;
98	I changed the variable names here a bit and checked the spline type and continued, just to decrease indentation a bit.