The section says:

"For all primitive types, enums, and
reference types, an in argument A of type P, that argument is passed as P. For all other types, an in argument A of type P is
passed as const P&. For an inout and out argument it is passed as P&. If we return a type of P, it is returned as P."

This is the only use of "primitive types" in the spec. "Basic Data Types" should be preferred. Would suggest:

"For all Basic Data Types [link/ref to 6.6 Mapping for Basic Data Types], enums [link/ref to 6.9 Mapping for Enums], and
reference types [link/ref to 6.7.1 Reference Types], an in argument A of type P, that argument is passed as P. For all other types, an in argument A of type P is
passed as const P&. For an inout and out argument it is passed as P&. If we return a type of P, it is returned as P."

using M2 = IDL::bounded_map<std::string, T, 20;

should be

using M2 = IDL::bounded_map<std::string, T, 20>;