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"Ambiguous implicit values" errors that make no sense

See original GitHub issue

Issue Description

I am getting seemingly random error messages like

ambiguous implicit values:  both value derive$macro$121 of type => upickle.default.Writer[models.AdminFoodRecord]  and value derive$macro$137 of type => upickle.default.Writer[uk.ac.ncl.openlab.intake24.InheritableAttributes]  match expected type upickle.default.Writer[T1]
ambiguous implicit values:  both value derive$macro$190 of type => upickle.default.Writer[models.AdminFoodRecord]  and value derive$macro$206 of type => upickle.default.Writer[uk.ac.ncl.openlab.intake24.InheritableAttributes]  match expected type upickle.default.Writer[T1]
not enough arguments for method Tuple4W: (implicit evidence$13: upickle.default.Writer[T1], implicit evidence$14: upickle.default.Writer[T2], implicit evidence$15: upickle.default.Writer[T3], implicit evidence$16: upickle.default.Writer[T4])upickle.default.Writer[(T1, T2, T3, T4)]. Unspecified value parameters evidence$13, evidence$14, evidence$15...    
uPickle does not know how to write [T1]s; define an implicit Writer[T1] to teach it how

These are very confusing since the types that supposedly match the expected type are unrelated, so there is no way they can both match.

Also, this part: uPickle does not know how to write [T1]s; define an implicit Writer[T1] to teach it how – T1 does not seem to be a real type, just a placeholder name.

How can I make sense of this? In particular, how can I tell what is the actual type of T1?