CVS difference for ai12s/ai12-0036-1.txt

--- ai12s/ai12-0036-1.txt	2012/11/29 06:43:27	1.1
+++ ai12s/ai12-0036-1.txt	2013/12/07 05:41:41	1.2
@@ -1,9 +1,12 @@
-!standard 12.5.1(5.1/3)                                   12-11-29    AI12-0036-1/01
+!standard 12.5.1(5.1/3)                                   13-12-06    AI12-0036-1/02
 !class binding interpretation 12-11-29
+!status Corrigendum 2014 13-12-06
+!status ARG Approved 9-0-2  13-11-16
 !status work item 12-11-29
 !status received 12-08-01
 !priority Low
 !difficulty Medium
+!qualifier Omission
 !subject The actual for an untagged formal derived type cannot be tagged
 !summary 
 
@@ -14,25 +17,25 @@
 Consider:
 
     generic
-        type T1 is private;
-        type T2 is new T1;
-        type T3 is new T1;
+       type T1 is private;
+       type T2 is new T1;
+       type T3 is new T1;
     package Pack1 is
-        procedure Proc;
+       procedure Proc;
     end Pack1;
 
     package body Pack1 is
-        
-        V1 : T1;
-        V2 : T2;
-        V3 : T3;
-
-        procedure Proc is
-        begin
-            V2 := T2(V1);  -- (A)
-            V3 := T3(V2);  -- (B)
-            V1 := T1(V2);  -- (C)
-        end Proc;
+    
+       V1 : T1;
+       V2 : T2;
+       V3 : T3;
+
+       procedure Proc is
+       begin
+          V2 := T2(V1);  -- (A)
+          V3 := T3(V2);  -- (B)
+          V1 := T1(V2);  -- (C)
+       end Proc;
 
     end Pack1;
 
@@ -59,9 +62,9 @@
 
 The actual type for a formal derived type shall be a descendant of the
 ancestor type and every progenitor of the formal type. If the formal type is
-nonlimited, the actual type shall be nonlimited. {If the actual type for
-a formal derived type is tagged, the formal derived type shall be a
-private extension.} If the reserved word synchronized appears in the
+nonlimited, the actual type shall be nonlimited. {The actual type for a
+formal derived type shall be tagged if and only if the formal derived type
+is a private extension.} If the reserved word synchronized appears in the
 declaration of the formal derived type, the actual type shall be a
 synchronized tagged type.
 
@@ -83,32 +86,88 @@
 In addition, untagged formal derived types are rare, so compatibility issues
 are likely to be much rarer.
 
+This solution is still incompatible, but only in cases where the formal derived
+type does not accurately describe the actual. As noted above, we believe such
+instances will be rare.
+
 Note that the problem does not require the types to be declared in the same
 generic unit; the problem also appears in cases like:
 
     generic
-        type T1 is private;
+       type T1 is private;
     package Pack1 is
-        generic
-            type T2 is new T1;
-            type T3 is new T1;
-        package Inner_Generic is ... 
+       generic
+          type T2 is new T1;
+          type T3 is new T1;
+       package Inner_Generic is ... 
     end Pack1;
 
 or
     
     generic
-        type T1 is private;
+       type T1 is private;
     package Pack1 is ...
     end Pack1;
     
     generic
-        type T2 is new T1;
-        type T3 is new T1;
+       type T2 is new T1;
+       type T3 is new T1;
     package Pack1.Child is ...
     end Pack1.Child;
 
 The matching rule works in these cases as well.
+
+We make the rule symmetric as it is possible to match a formal private
+extension with a nontagged type:
+
+    generic
+       type N is tagged private;
+       type NC is new N with private;
+    package GG is
+       ...
+    end GG;
+
+    package P is
+       type T1 is private;
+    private 
+       type T1 is tagged ...
+    end P;
+
+    with P;
+    package Q is
+       type T2 is new P.T1;
+    end Q;
+
+    with Q;
+    package body P is
+       package G is new GG (T1, T2);
+       ...
+    end P;
+
+Here, T1 is tagged, but T2 is not tagged nor an extension. It's not
+clear that there is any semantic problem with allowing a case like
+this, but we're taking the safest option here.
+
+!corrigendum 12.5.1(5.1/3)
+
+@drepl
+The actual type for a formal derived type shall be a descendant of the ancestor
+type and every progenitor of the formal type. If the formal type is nonlimited,
+the actual type shall be nonlimited. If the reserved word @b<synchronized>
+appears in the declaration of the formal derived type, the actual type shall be a
+synchronized tagged type.
+@dby
+The actual type for a formal derived type shall be a descendant of the
+ancestor type and every progenitor of the formal type. If the formal type is
+nonlimited, the actual type shall be nonlimited. The actual type for a
+formal derived type shall be tagged if and only if the formal derived type
+is a private extension. If the reserved word @b<synchronized> appears in the
+declaration of the formal derived type, the actual type shall be a
+synchronized tagged type.
+
+!ASIS
+
+No ASIS impact.
 
 !ACATS test