implement support for node " N_Expanded_Name"

[sonodtt]

Summary: renaming a package variable fails
– currently unsupported:
Var : ET.New_Integer renames ET.External_Var;

This task lies somewhere between "support for renaming" and "support for packages".

Consider the test case below

procedure Declaration_Rename is
A : Integer := 1;
B : Integer;

New_Integer_With_Complicated_Name : Integer := 11;
Simply_Named_Int : Integer renames New_Integer_With_Complicated_Name;
function My_Plus(Left, Right : Integer) return Integer renames "+";

package ET renames External_Types;
– without a 'use' package clause (which would make the package renaming redundant)
– the operator must also be explicitly renamed 
– (this operator is required for the 'pragma Assert'
function "=" (Left, Right: ET.New_Integer) return Boolean renames ET."=";

– currently unsupported. 
Var : ET.New_Integer renames ET.External_Var;

C : ET.New_Integer := 4;
begin
A := A + 1;
B := A + 1;

Var := 5;

pragma Assert (B=3);
pragma Assert (Simply_Named_Int=11);
pragma Assert (My_Plus(A,B)=5);

pragma Assert (C=4);

pragma Assert (Var=4);
end Declaration_Rename;

All operations involving Var (in the example above an assignment statement and equality expression) will cause failure in 'Do_Expression' with report:

12:42 $ gnat2goto declaration_rename.adb
---------At: Do_Expression---------
---------Unknown expression kind---------
N_Expanded_Name "external_var" (Node_Id=2406) (analyzed)
Rewritten: original node = N_Identifier "var" (Node_Id=2547)
Parent = N_Assignment_Statement (Node_Id=2408)
Sloc = 13314 declaration_rename.adb:31:4
Chars = "external_var" (Name_Id=300001456)
Etype = N_Defining_Identifier "new_integer" (Entity_Id=2455)
Prefix = N_Identifier "et" (Node_Id=2546)
Selector_Name = N_Identifier "external_var" (Node_Id=2545)
Entity = N_Defining_Identifier "external_var" (Entity_Id=2465)
Associated_Node = N_Defining_Identifier "external_var" (Entity_Id=2465)
---------At: Do_Expression---------
---------Unknown expression kind---------
N_Expanded_Name "external_var" (Node_Id=2438) (analyzed)
Rewritten: original node = N_Identifier "var" (Node_Id=2589)
Parent = N_Op_Eq "Oeq" (Node_Id=2441)
Sloc = 13477 declaration_rename.adb:39:19
Chars = "external_var" (Name_Id=300001456)
Etype = N_Defining_Identifier "new_integer" (Entity_Id=2455)
Prefix = N_Identifier "et" (Node_Id=2588)
Selector_Name = N_Identifier "external_var" (Node_Id=2587)
Entity = N_Defining_Identifier "external_var" (Entity_Id=2465)
Associated_Node = N_Defining_Identifier "external_var" (Entity_Id=2465)

The issue concerns also getting a fully qualified name:

-------------------
– Expanded Name –
-------------------

– The N_Expanded_Name node is used to represent a selected component
– name that has been resolved to an expanded name. The semantic phase
– replaces N_Selected_Component nodes that represent names by the use
– of this node, leaving the N_Selected_Component node used only when
– the prefix is a record or protected type.

– The fields of the N_Expanded_Name node are layed out identically
– to those of the N_Selected_Component node, allowing conversion of
– an expanded name node to a selected component node to be done
– easily, see Sinfo.CN.Change_Selected_Component_To_Expanded_Name.

– There is no special sprint syntax for an expanded name

– N_Expanded_Name
– Sloc points to the period
– Chars (Name1) copy of Chars field of selector name
– Prefix (Node3)
– Selector_Name (Node2)
– Entity (Node4-Sem)
– Associated_Node (Node4-Sem)
– Has_Private_View (Flag11-Sem) set in generic units.
– Redundant_Use (Flag13-Sem)
– Atomic_Sync_Required (Flag14-Sem)
– plus fields for expression

The prefix here is apparently what we'd need for a FQN, but it is uncertain if just that much will be enough to get it to work;