The "function descriptor" terminology was inspired by the JVM Spec's use of "descriptor" to refer to a parameter/return signature of a method. In retrospect, this re-use of the term is confusing and compounds existing problems with the VM vs. language terminology in this space (e.g., "signature" in JLS vs. JVMS).

To clear this up and better align with user intuition:
- A "function type" is a "method type" (see JLS 8.2) -- not a type itself, but a tuple of types and type lists
- A functional interface "has" a function type
- The types that are valid lambda targets, including intersections, are referred to as "functional interface types"