[Ada] Fix expansion of attributes Input/Output for unchecked union types

[Ada] Fix expansion of attributes Input/Output for unchecked union types

[Pierre-Marie de Rodat]

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Expansion of attributes Input/Output for unchecked unions didn't work,
because it looked for a No_Elist where the AST has an empty list. Now we
look for a default value of a first discriminant (because the default
values must be given either for all or for none of the discrminants).

Also, it was wrong to insert an N_Raise_Program_Error as an action and
leave the original N_Attribute_Reference node, because Gigi doesn't know
how to expand input or output attribute references. Now we rewrite the
attribute reference just like a neighbouring code which raises
Program_Error with a reason of PE_Stream_Operation_Not_Allowed.

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

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Fix expansion of
	attributes Input and Output for unchecked unions.
	* sem_case.ads: Fix typo "disriminant" and refill comment.

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diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -4237,12 +4237,13 @@ package body Exp_Attr is
                --  type if the type lacks default discriminant values.
 
                if Is_Unchecked_Union (Base_Type (U_Type))
-                 and then No (Discriminant_Constraint (U_Type))
+                 and then
+                 No (Discriminant_Default_Value (First_Discriminant (U_Type)))
                then
-                  Insert_Action (N,
+                  Rewrite (N,
                     Make_Raise_Program_Error (Loc,
                       Reason => PE_Unchecked_Union_Restriction));
-
+                  Set_Etype (N, B_Type);
                   return;
                end if;
 
@@ -5332,12 +5333,13 @@ package body Exp_Attr is
                --  values.
 
                if Is_Unchecked_Union (Base_Type (U_Type))
-                 and then No (Discriminant_Constraint (U_Type))
+                 and then
+                 No (Discriminant_Default_Value (First_Discriminant (U_Type)))
                then
-                  Insert_Action (N,
+                  Rewrite (N,
                     Make_Raise_Program_Error (Loc,
                       Reason => PE_Unchecked_Union_Restriction));
-
+                  Set_Etype (N, Standard_Void_Type);
                   return;
                end if;
 


diff --git a/gcc/ada/sem_case.ads b/gcc/ada/sem_case.ads
--- a/gcc/ada/sem_case.ads
+++ b/gcc/ada/sem_case.ads
@@ -44,8 +44,8 @@
 --    till the freeze point in this case.
 
 --    3. Set the Others_Discrete_Choices list for an others choice. This is
---    used in various ways, e.g. to construct the disriminant checking function
---    for the case of a variant with an others choice.
+--    used in various ways, e.g. to construct the discriminant checking
+--    function for the case of a variant with an others choice.
 
 --    4. In the case of static predicates, we need to expand out choices that
 --    correspond to the predicate for the back end. This expansion destroys