[Ada] Crash on constrained container in generalized indexing operation

[Ada] Crash on constrained container in generalized indexing operation

2019-12-16  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch4.adb (Try_Container_Indexing): In the case of a derived
	container type, use the base type to look for candidate indexing
	operations, because the object may be a constrained subtype or
	itype with no explicit declaration.  Candidate indexing
	operations are found in the same scope and list of declarations
	as the declaration of the base type.

From-SVN: r279432