[Ada] Attribute Img on derived types

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

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Refinement of previous patch: the root type must be used for the image
of enumeration types, because the literal map is attached to the root
type even when derivations are present.
Fixes several ACATS and fixedbugs regressions involving 'Image.

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

gcc/ada/

	*  exp_imgv.adb (Expand_Image_Attribute):  Refine previous patch
	to use root type (and not base type) on enumeration types.

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diff --git a/gcc/ada/exp_imgv.adb b/gcc/ada/exp_imgv.adb
--- a/gcc/ada/exp_imgv.adb
+++ b/gcc/ada/exp_imgv.adb
@@ -481,9 +481,15 @@ package body Exp_Imgv is
 
       --  Ada 2020 allows 'Image on private types, so fetch the underlying
       --  type to obtain the structure of the type. We use the base type,
-      --  not the root type, to handle properly derived types.
+      --  not the root type, to handle properly derived types, but we use
+      --  the root type for enumeration types, because the literal map is
+      --  attached to the root. Should be inherited ???
 
-      Rtyp := Underlying_Type (Base_Type (Ptyp));
+      if Is_Enumeration_Type (Ptyp) then
+         Rtyp := Underlying_Type (Root_Type (Ptyp));
+      else
+         Rtyp := Underlying_Type (Base_Type (Ptyp));
+      end if;
 
       --  Enable speed-optimized expansion of user-defined enumeration types
       --  if we are compiling with optimizations enabled and enumeration type