[Ada] bugbox referencing INOX prefixed view of primitive op of modular type

["Marc Poulhiès" <poulhies@adacore.com>]

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The -gnatX switch enables naming a prefixed view of a subprogram with an
untagged prefix. Compiling such a reference where the prefix is of a modular
type could result in an internal compiler error.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch4.adb
	(Analyze_Selected_Component): Avoid initializing the local
	variable Comp if the variable is not going to be subsequently
	referenced. This is a correctness issue because the call to
	First_Entity can fail.

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diff --git a/gcc/ada/sem_ch4.adb b/gcc/ada/sem_ch4.adb
--- a/gcc/ada/sem_ch4.adb
+++ b/gcc/ada/sem_ch4.adb
@@ -5187,7 +5187,16 @@ package body Sem_Ch4 is
           and then not Is_Derived_Type (Prefix_Type)
           and then Is_Entity_Name (Name);
 
-      Comp := First_Entity (Type_To_Use);
+      --  Avoid initializing Comp if that initialization is not needed
+      --  (and, more importantly, if the call to First_Entity could fail).
+
+      if Has_Discriminants (Type_To_Use)
+        or else Is_Record_Type (Type_To_Use)
+        or else Is_Private_Type (Type_To_Use)
+        or else Is_Concurrent_Type (Type_To_Use)
+      then
+         Comp := First_Entity (Type_To_Use);
+      end if;
 
       --  If the selector has an original discriminant, the node appears in
       --  an instance. Replace the discriminant with the corresponding one