ada: Missing runtime tag check on mutably tagged objects

This patch fixes an issue in the compiler whereby assigning to a non-existant
mutably tagged object component failed to result in the expected run-time
exception.

gcc/ada/ChangeLog:

	* exp_ch4.adb (Expand_N_Type_Conversion): Add special runtime check
	generation for mutably tagged objects.

1 files changed, 30 insertions, 2 deletions

diff --git a/gcc/ada/exp_ch4.adb b/gcc/ada/exp_ch4.adb
index 381c9f8..c16e09d 100644
--- a/gcc/ada/exp_ch4.adb
+++ b/gcc/ada/exp_ch4.adb
@@ -11684,6 +11684,34 @@ package body Exp_Ch4 is
          end if;
       end if;
 
+      --  Generate a tag check for view conversions of mutably tagged objects,
+      --  which are special in nature and require selecting the tag component
+      --  from the class-wide equivalent type.
+
+      --  Possibly this could be combined with the logic below for better code
+      --  reuse ???
+
+      if Is_View_Conversion (N)
+        and then Is_Variable (Operand)
+        and then Is_Class_Wide_Equivalent_Type (Etype (Operand))
+      then
+         --  Generate:
+         --    [Constraint_Error when Operand.Tag /= Root_Type]
+
+         Insert_Action (N,
+           Make_Raise_Constraint_Error (Loc,
+             Condition =>
+               Make_Op_Ne (Loc,
+                 Left_Opnd  =>
+                   Make_Selected_Component (Loc,
+                     Prefix        => Duplicate_Subexpr_No_Checks (Operand),
+                     Selector_Name => Make_Identifier (Loc, Name_uTag)),
+                 Right_Opnd =>
+                   Make_Attribute_Reference (Loc,
+                     Prefix         => New_Occurrence_Of (Target_Type, Loc),
+                     Attribute_Name => Name_Tag)),
+             Reason   => CE_Tag_Check_Failed));
+
       --  Case of conversions of tagged types and access to tagged types
 
       --  When needed, that is to say when the expression is class-wide, Add
@@ -11699,8 +11727,8 @@ package body Exp_Ch4 is
       --          and then Operand.all not in
       --            Designated_Type (Target_Type)'Class]
 
-      if (Is_Access_Type (Target_Type)
-           and then Is_Tagged_Type (Designated_Type (Target_Type)))
+      elsif (Is_Access_Type (Target_Type)
+              and then Is_Tagged_Type (Designated_Type (Target_Type)))
         or else Is_Tagged_Type (Target_Type)
       then
          --  Do not do any expansion in the access type case if the parent is a