[Ada] Handle defaults in declare_expressions in postconditions

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

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In Ada2020 the expression for a postcondition can be a
Declare_Expression containing object declarations with defaults. These
defaults may generate local subtypes, but as for other default
expressions their full expansion must be deferred, typically to the
freeze point of the object being declared, which for a postcondition
will be the analysis of the generated postcondition procedure.
Nevertheless the declarations for such subtypes must appear in the tree
to complete the semantic analysis of the default expression itself.
Given that Insert_Actions is disabled when In_Spec_Expression is set, we
place the generated declaration directly ahead of the local declaration,
and provide the needed semantic information for the subtype.

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

gcc/ada/

	* sem_ch3.adb (Find_Type_Of_Object):  When In_Spec_Expression is
	set and the object declaration generates a subtype indication,
	build the corresponding subtype declaration and place it in tree
	without the use of Insert_Actions, which is disabled in this
	context.

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diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -17827,6 +17827,44 @@ package body Sem_Ch3 is
 
          T := Make_Defining_Identifier (Sloc (P), Nam);
 
+         --  If In_Spec_Expression, for example within a pre/postcondition,
+         --  provide enough information for use of the subtype without
+         --  depending on full analysis and freezing, which will happen when
+         --  building the correspondiing subprogram.
+
+         if In_Spec_Expression then
+            Analyze (Subtype_Mark (Obj_Def));
+
+            declare
+               Base_T : constant Entity_Id := Entity (Subtype_Mark (Obj_Def));
+               Decl   : constant Node_Id :=
+                 Make_Subtype_Declaration (Sloc (P),
+                   Defining_Identifier => T,
+                   Subtype_Indication  => Relocate_Node (Obj_Def));
+            begin
+               Set_Etype  (T, Base_T);
+               Set_Ekind  (T, Subtype_Kind (Ekind (Base_T)));
+               Set_Parent (T, Obj_Def);
+
+               if Ekind (T) = E_Array_Subtype then
+                  Set_First_Index (T, First_Index (Base_T));
+                  Set_Is_Constrained (T);
+
+               elsif Ekind (T) = E_Record_Subtype then
+                  Set_First_Entity (T, First_Entity (Base_T));
+                  Set_Has_Discriminants (T, Has_Discriminants (Base_T));
+                  Set_Is_Constrained (T);
+               end if;
+
+               Insert_Before (Related_Nod, Decl);
+            end;
+
+            return T;
+         end if;
+
+         --  When generating code, insert subtype declaration ahead of
+         --  declaration that generated it.
+
          Insert_Action (Obj_Def,
            Make_Subtype_Declaration (Sloc (P),
              Defining_Identifier => T,