[Ada] Implementation of AI12-0125, use of @ as abbreviation for LHS.

[Ada] Implementation of AI12-0125, use of @ as abbreviation for LHS.

[Arnaud Charlet]

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With this patch the compiler now handles properly the use of @ as a prefix
of a reference to a discriminated record component and to its bounds.

The following must compile quietly in gnat2020 mode:

---
procedure Discrs is
begin
   declare
    -- Discrim-dependent subtypes
    subtype Index is Integer range 0 .. 123;
    type R1 (D1 : Index := 0) is record F1 : String (1 .. D1); end record;
    type R2 (D2 : Index := 0) is record F2 : R1 (D1 => D2); end record;
    X : R2;
   begin
     for I in 1 .. Index'Last loop
        X := (I, (I, (others => 'A')));
        X.F2.F1 (1) := 'B';
        X.F2.F1:= 'C' & @ (@'First .. @'Last - 1);
        X.F2 := (@.D1,  'D' & @.F1 (@.F1'First .. @.F1'Last - 1));
        X := (D2 => @.D2,
              F2 =>
                (D1 => @.D2,
                 F1 => 'E' & @.F2.F1 (@.F2.F1'First .. @.F2.F1'Last - 1)));
        pragma Assert
          (X.F2.F1 = (if I <= 4
                      then String'("EDCB")(1..I)
                      else "EDCB" & (1 .. I-4 => 'A')));
     end loop;
   end;
end;

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

2017-04-25  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch5.adb (Analyze_Assignment): Reset Full_Analysis flag on
	the first pass over an assignment statement with target names,
	to prevent the generation of subtypes (such as discriminated
	record components)that may carry the target name outside of the
	tree for the assignment. The subtypes will be generated when
	the assignment is reanalyzed in full.
	(Analyze_Target_Name): Handle properly class-wide types.


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Index: sem_ch5.adb
===================================================================
--- sem_ch5.adb	(revision 247146)
+++ sem_ch5.adb	(working copy)
@@ -64,10 +64,12 @@
 
 package body Sem_Ch5 is
 
-   Current_LHS : Node_Id := Empty;
-   --  Holds the left-hand side of the assignment statement being analyzed.
-   --  Used to determine the type of a target_name appearing on the RHS, for
-   --  AI12-0125 and the use of '@' as an abbreviation for the LHS.
+   Current_Assignment : Node_Id := Empty;
+   --  This variable holds the node for an assignment that contains target
+   --  names. The corresponding flag has been set by the parser, and when
+   --  set the analysis of the RHS must be done with all expansion disabled,
+   --  because the assignment is reanalyzed after expansion has replaced all
+   --  occurrences of the target name appropriately.
 
    Unblocked_Exit_Count : Nat := 0;
    --  This variable is used when processing if statements, case statements,
@@ -98,11 +100,12 @@
    --  Ghost mode.
 
    procedure Analyze_Assignment (N : Node_Id) is
-      Lhs  : constant Node_Id := Name (N);
-      Rhs  : constant Node_Id := Expression (N);
-      T1   : Entity_Id;
-      T2   : Entity_Id;
-      Decl : Node_Id;
+      Lhs                : constant Node_Id := Name (N);
+      Rhs                : constant Node_Id := Expression (N);
+      T1                 : Entity_Id;
+      T2                 : Entity_Id;
+      Decl               : Node_Id;
+      Save_Full_Analysis : Boolean;
 
       procedure Diagnose_Non_Variable_Lhs (N : Node_Id);
       --  N is the node for the left hand side of an assignment, and it is not
@@ -284,10 +287,6 @@
    --  Start of processing for Analyze_Assignment
 
    begin
-      --  Save LHS for use in target names (AI12-125)
-
-      Current_LHS := Lhs;
-
       Mark_Coextensions (N, Rhs);
 
       --  Analyze the target of the assignment first in case the expression
@@ -301,7 +300,12 @@
       --  during analysis and expansion are properly marked as Ghost.
 
       if Has_Target_Names (N) then
+         Current_Assignment := N;
          Expander_Mode_Save_And_Set (False);
+         Save_Full_Analysis := Full_Analysis;
+         Full_Analysis      := False;
+      else
+         Current_Assignment := Empty;
       end if;
 
       Mark_And_Set_Ghost_Assignment (N, Mode);
@@ -932,7 +936,6 @@
       Analyze_Dimension (N);
 
    <<Leave>>
-      Current_LHS := Empty;
       Restore_Ghost_Mode (Mode);
 
       --  If the right-hand side contains target names, expansion has been
@@ -942,6 +945,7 @@
 
       if Nkind (N) = N_Assignment_Statement and then Has_Target_Names (N) then
          Expander_Mode_Restore;
+         Full_Analysis := Save_Full_Analysis;
       end if;
    end Analyze_Assignment;
 
@@ -3543,14 +3547,10 @@
 
    procedure Analyze_Target_Name (N : Node_Id) is
    begin
-      if No (Current_LHS) then
-         Error_Msg_N ("target name can only appear within an assignment", N);
-         Set_Etype (N, Any_Type);
+      --  A target name has the type of the left-hand side of the enclosing
+      --  assignment.
 
-      else
-         Set_Has_Target_Names (Parent (Current_LHS));
-         Set_Etype (N, Etype (Current_LHS));
-      end if;
+      Set_Etype (N, Etype (Name (Current_Assignment)));
    end Analyze_Target_Name;
 
    ------------------------