[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

gcc/ada/ChangeLog

 2019-12-16  Ed Schonberg  <schonberg@adacore.com>
 
+	* 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.
+
+2019-12-16  Ed Schonberg  <schonberg@adacore.com>
+
 	* scng.adb (Scan): In Ada2020, a left-bracket indicates the
 	start of an homogenous aggregate.
 	* par-ch4.adb (P_Reduction_Attribute_Reference): New procedure.

gcc/ada/sem_ch4.adb

@@ -8289,13 +8289,16 @@ package body Sem_Ch4 is
       --  Note that predefined containers are typically all derived from one of
       --  the Controlled types. The code below is motivated by containers that
       --  are derived from other types with a Reference aspect.
+      --  Note as well that we need to examine the base type, given that
+      --  the container object may be a constrained subtype or itype which
+      --  does not have an explicit declaration,
 
       elsif Is_Derived_Type (C_Type)
         and then Etype (First_Formal (Entity (Func_Name))) /= Pref_Typ
       then
          Func_Name :=
            Find_Indexing_Operations
-             (T           => C_Type,
+             (T           => Base_Type (C_Type),
               Nam         => Chars (Func_Name),
               Is_Constant => Is_Constant_Indexing);
       end if;