[Ada] Inconsistent compile time Constraint_Error warning

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch corrects several bugs within the compiler which led to
inconsistent handling of compile time Constraint_Errors. Notibly,
subtype out of range checks which are only out of range of the subtype
must be warnings while out of range checks where the value is out of
range of the base type must be an error. Also, type conversions and
qualified expressions on literals constitute errors on any out of range
value. The compiler needed many of these cases clarified.

------------
-- Source --
------------

--  main.ads

with System;
package Main is

   type T_Enum is (Enum_1, Enum_2, Unknown)
     with Default_Value => Unknown;

   subtype T_Valid_Enum is T_Enum range Enum_1 .. Enum_2;

   Value : T_Valid_Enum; --  WARNING

   generic
      type T_Element is (<>);
      Init : T_Element;
   package Generic_Test is
      Value : T_Element := Init;
   end;

   package titi is new Generic_Test (T_Valid_Enum, Unknown); --  WARNING

   type My_Float is digits System.Max_Base_Digits;

   My_Float_Last : constant := My_Float'Last;
   Out_Of_Range  : constant := My_Float_Last + 1.0;

   Flt1 : My_Float := Out_Of_Range; --  ERROR

   A : Positive := Positive (16#9999_9999_9999#); --  ERROR
   B : Positive := 16#9999_9999_9999#;            --  ERROR
   C : Positive := 0;                             --  WARNING
   D : Positive := Positive (0);                  --  ERROR
   E : Positive := Positive'(16#9999_9999_9999#); --  ERROR
   F : Positive := Positive'(0);                  --  ERROR
end;

-----------------
-- Compilation --
-----------------

$ gnatmake -q -gnatw_a main.adb
main.ads:9:12: warning: value not in range of type "T_Valid_Enum" defined at
line 7
main.ads:9:12: warning: "Constraint_Error" will be raised at run time
main.ads:18:52: warning: value not in range of type "T_Element" defined at
line 12, instance at line 18
main.ads:18:52: warning: "Constraint_Error" will be raised at run time
main.ads:25:23: value not in range of type "My_Float" defined at line 20
main.ads:25:23: static expression fails Constraint_Check
main.ads:27:19: value not in range of type "Standard.Positive"
main.ads:27:19: static expression fails Constraint_Check
main.ads:28:19: value not in range of type "Standard.Positive"
main.ads:28:19: static expression fails Constraint_Check
main.ads:29:19: warning: value not in range of type "Standard.Positive"
main.ads:29:19: warning: "Constraint_Error" will be raised at run time
main.ads:30:19: value not in range of type "Standard.Positive"
main.ads:30:19: static expression fails Constraint_Check
main.ads:31:27: value not in range of type "Standard.Positive"
main.ads:31:27: static expression fails Constraint_Check
main.ads:32:27: value not in range of type "Standard.Positive"
main.ads:32:27: static expression fails Constraint_Check
gnatmake: "main.ads" compilation error

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

2019-08-12  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_eval.adb (Check_Non_Static_Context): Add a condition to
	determine if a range violation constitues a warning or an error.
	(Out_Of_Range): Add a condition to determine if a range
	violation constitues a warning or an error.

--- gcc/ada/sem_eval.adb
+++ gcc/ada/sem_eval.adb
@@ -562,23 +562,31 @@ package body Sem_Eval is
       elsif Is_Out_Of_Range (N, Base_Type (T), Assume_Valid => True) then
          Out_Of_Range (N);
 
-      --  Give warning if outside subtype (where one or both of the bounds of
-      --  the subtype is static). This warning is omitted if the expression
-      --  appears in a range that could be null (warnings are handled elsewhere
-      --  for this case).
+      --  Give a warning or error on the value outside the subtype. A
+      --  warning is omitted if the expression appears in a range that could
+      --  be null (warnings are handled elsewhere for this case).
 
       elsif T /= Base_Type (T) and then Nkind (Parent (N)) /= N_Range then
          if Is_In_Range (N, T, Assume_Valid => True) then
             null;
 
          elsif Is_Out_Of_Range (N, T, Assume_Valid => True) then
-
             --  Ignore out of range values for System.Priority in CodePeer
             --  mode since the actual target compiler may provide a wider
             --  range.
 
             if CodePeer_Mode and then T = RTE (RE_Priority) then
                Set_Do_Range_Check (N, False);
+
+            --  Determine if the out of range violation constitutes a warning
+            --  or an error based on context according to RM 4.9 (34/3).
+
+            elsif Nkind_In (Original_Node (N), N_Type_Conversion,
+                                               N_Qualified_Expression)
+              and then Comes_From_Source (Original_Node (N))
+            then
+               Apply_Compile_Time_Constraint_Error
+                 (N, "value not in range of}", CE_Range_Check_Failed);
             else
                Apply_Compile_Time_Constraint_Error
                  (N, "value not in range of}<<", CE_Range_Check_Failed);
@@ -5515,8 +5523,18 @@ package body Sem_Eval is
          --  CodePeer mode where the target runtime may have more priorities.
 
          elsif not CodePeer_Mode or else Etype (N) /= RTE (RE_Priority) then
-            Apply_Compile_Time_Constraint_Error
-              (N, "value not in range of}", CE_Range_Check_Failed);
+            --  Determine if the out of range violation constitutes a warning
+            --  or an error based on context according to RM 4.9 (34/3).
+
+            if Nkind (Original_Node (N)) = N_Type_Conversion
+              and then not Comes_From_Source (Original_Node (N))
+            then
+               Apply_Compile_Time_Constraint_Error
+                 (N, "value not in range of}??", CE_Range_Check_Failed);
+            else
+               Apply_Compile_Time_Constraint_Error
+                 (N, "value not in range of}", CE_Range_Check_Failed);
+            end if;
          end if;
 
       --  Here we generate a warning for the Ada 83 case, or when we are in an