[Ada] Fix dispatching call to primitive function with controlling tagged result

[Pierre-Marie de Rodat <derodat@adacore.com>]

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When a dispatching call is made to a primitive function with a controlling
tagged result, the call is dispatching on result and thus must return the
class-wide type of the tagged type to accommodate all possible results.

This was ensured by Expand_Dispatching_Call only in the common case where
the result type is the type of the controlling argument, which does not
cover the case of a primitive function inherited from an ancestor type.

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

gcc/ada/

	* exp_disp.adb (Expand_Dispatching_Call): Fix detection of calls
	that are dispatching on tagged result.

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diff --git a/gcc/ada/exp_disp.adb b/gcc/ada/exp_disp.adb
--- a/gcc/ada/exp_disp.adb
+++ b/gcc/ada/exp_disp.adb
@@ -896,8 +896,14 @@ package body Exp_Disp is
       Copy_Strub_Mode (Subp_Typ, Subp);
       Set_Convention  (Subp_Typ, Convention (Subp));
 
-      if Etype (Subp) = Typ then
-         Set_Etype          (Subp_Typ, CW_Typ);
+      --  If this is a function and it has a controlling tagged result, then
+      --  the call is dispatching on result and returns the class-wide type.
+
+      if Ekind (Subp) = E_Function
+        and then Has_Controlling_Result (Subp)
+        and then Is_Tagged_Type (Etype (Subp))
+      then
+         Set_Etype          (Subp_Typ, Class_Wide_Type (Etype (Subp)));
          Set_Returns_By_Ref (Subp_Typ, True);
       else
          Set_Etype          (Subp_Typ, Etype (Subp));