[Ada] Spurious error on overriding protected function in instance

[Ada] Spurious error on overriding protected function in instance

The conformance between an overriding protected operation with
progenitors and the overridden interface operation requires subtype
conformance; requiring equality of return types in the case of a
function is too restrictive and leads to spurious errors when the return
type is a generic actual.

2018-08-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch6.adb (Check_Synchronized_Overriding): The conformance
	between an overriding protected operation and the overridden
	abstract progenitor operation requires subtype conformance;
	requiring equality of return types in the case of a function is
	too restrictive and leads to spurious errors when the return
	type is a generic actual.

gcc/testsuite/

	* gnat.dg/prot6.adb, gnat.dg/prot6.ads: New testcase.

From-SVN: r263731