sem_ch4.adb (Try_Primitive_Operations, [...]): argument is valid if it is a derived type with unknown discriminants that...

2017-01-23  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch4.adb (Try_Primitive_Operations,
	Is_Valid_First_Argument_Of): argument is valid if it is a derived
	type with unknown discriminants that matches its underlying
	record view.
	* exp_util.adb (Expand_Subtype_From_Expr): Do not expand
	expression if its type is derived from a limited type with
	unknown discriminants, because the expansion (which is a call)
	must be expanded in the enclosing context to add the proper build-
	in-place parameters to the call.
	* lib.ads, exp_ch9.adb: Minor fixes in comments.

From-SVN: r244790