[COMMITTED] ada: Illegal instance of Generic_1.Generic_2 incorrectly accepted

["Marc Poulhiès" <poulhies@adacore.com>]

From: Steve Baird <baird@adacore.com>

If G1 is a generic package and G1.G2 is a child unit (also a generic package)
then it would be illegal if some third generic unit (declared outside of G1)
takes a formal instance of G1.G2, as in "with package I2 is new G1.G2;".
This construct was incorrectly accepted in some cases.

gcc/ada/

	* sem_ch12.adb (Check_Generic_Child_Unit): Introduce a new nested
	function Adjusted_Inst_Par_Ekind to cope with cases where either
	a- the visibility of a compiler-generated renaming is incorrect;
	or b- we are inside of a generic parent unit G1 that has a child
	unit G1.G2, so instantiation of G1.G2 is permitted.

Tested on x86_64-pc-linux-gnu, committed on master.

---
 gcc/ada/sem_ch12.adb | 96 +++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 85 insertions(+), 11 deletions(-)

diff --git a/gcc/ada/sem_ch12.adb b/gcc/ada/sem_ch12.adb
index 5db9754f81d..e229d217555 100644
--- a/gcc/ada/sem_ch12.adb
+++ b/gcc/ada/sem_ch12.adb
@@ -7234,7 +7234,7 @@ package body Sem_Ch12 is
       Loc      : constant Source_Ptr := Sloc (Gen_Id);
       Gen_Par  : Entity_Id := Empty;
       E        : Entity_Id;
-      Inst_Par : Entity_Id;
+      Inst_Par : Entity_Id := Empty;
       S        : Node_Id;
 
       function Find_Generic_Child
@@ -7440,16 +7440,90 @@ package body Sem_Ch12 is
                --  the instance of Gpar, so this is illegal. The test below
                --  recognizes this particular case.
 
-               if Is_Child_Unit (E)
-                 and then not Comes_From_Source (Entity (Prefix (Gen_Id)))
-                 and then (not In_Instance
-                            or else Nkind (Parent (Parent (Gen_Id))) =
-                                                         N_Compilation_Unit)
-               then
-                  Error_Msg_N
-                    ("prefix of generic child unit must be instance of parent",
-                      Gen_Id);
-               end if;
+               declare
+                  --  We want to reject the final instantiation in
+                  --    generic package G1 is end G1;
+                  --    generic package G1.G2 is end G1.G2;
+                  --    with G1; package I1 is new G1;
+                  --    with G1.G2; package I1.I2 is new G1.G2;
+                  --  because the use of G1.G2 should instead be either
+                  --  I1.G2 or simply G2. However, the tree that is built
+                  --  in this case is wrong. In the expanded copy
+                  --  of G2, we need (and therefore generate) a renaming
+                  --    package G1 renames I1;
+                  --  but this renaming should not participate in resolving
+                  --  this occurrence of the name "G1.G2"; unfortunately,
+                  --  it does. Rather than correct this error, we compensate
+                  --  for it in this function.
+                  --
+                  --  We also perform another adjustment here. If we are
+                  --  currently inside a generic package, then that
+                  --  generic package needs to be treated as a package.
+                  --  For example, if a generic Aaa declares a nested generic
+                  --  Bbb (perhaps as a child unit) then Aaa can also legally
+                  --  declare an instance of Aaa.Bbb.
+
+                  function Adjusted_Inst_Par_Ekind return Entity_Kind;
+
+                  -----------------------------
+                  -- Adjusted_Inst_Par_Ekind --
+                  -----------------------------
+
+                  function Adjusted_Inst_Par_Ekind return Entity_Kind is
+                     Prefix_Entity   : Entity_Id;
+                     Inst_Par_GP     : Node_Id;
+                     Inst_Par_Parent : Node_Id := Parent (Inst_Par);
+                  begin
+                     if Nkind (Inst_Par_Parent) = N_Defining_Program_Unit_Name
+                     then
+                        Inst_Par_Parent := Parent (Inst_Par_Parent);
+                     end if;
+
+                     Inst_Par_GP := Generic_Parent (Inst_Par_Parent);
+
+                     if Nkind (Gen_Id) = N_Expanded_Name
+                       and then Present (Inst_Par_GP)
+                       and then Ekind (Inst_Par_GP) = E_Generic_Package
+                     then
+                        Prefix_Entity := Entity (Prefix (Gen_Id));
+
+                        if Present (Prefix_Entity)
+                           and then not Comes_From_Source (Prefix_Entity)
+                           and then Nkind (Parent (Prefix_Entity)) =
+                                      N_Package_Renaming_Declaration
+                           and then Chars (Prefix_Entity) = Chars (Inst_Par_GP)
+                        then
+                           return E_Generic_Package;
+                        end if;
+                     end if;
+
+                     if Ekind (Inst_Par) = E_Generic_Package
+                       and then In_Open_Scopes (Inst_Par)
+                     then
+                        --  If we are inside a generic package then
+                        --  treat it as a package.
+                        return E_Package;
+                     end if;
+
+                     --  The usual path
+                     return Ekind (Inst_Par);
+                  end Adjusted_Inst_Par_Ekind;
+
+               begin
+                  if Is_Child_Unit (E)
+                    and then (No (Inst_Par)
+                              or else Adjusted_Inst_Par_Ekind =
+                                      E_Generic_Package)
+                    and then (not In_Instance
+                              or else Nkind (Parent (Parent (Gen_Id))) =
+                                        N_Compilation_Unit)
+                  then
+                     Error_Msg_N
+                       ("prefix of generic child unit must be " &
+                          "instance of parent",
+                        Gen_Id);
+                  end if;
+               end;
 
                if not In_Open_Scopes (Inst_Par)
                  and then Nkind (Parent (Gen_Id)) not in
-- 
2.43.0