[Ada] Warn on statically known empty loop caused by constraint

[Pierre-Marie de Rodat]

Before this commit, GNAT would not warn about empty loops caused by
constraints like in the following piece of code:

procedure tmp is
   type T is mod 3;
begin
   for I in T range -1..1 loop
      null;
   end loop;
end;

When the constrained type is modular, additional information about
computed bounds are printed in order to help the user understand the
cause of the warning. The above example prints the following messages:

warning: constrained range is null
warning: iterator has modular type "T", so the loop has bounds 2 .. 1

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

gcc/ada/

        * sem_ch5.adb (Analyze_Loop_Parameter_Specification): Check for
        empty loops caused by constraints.
diff --git a/gcc/ada/sem_ch5.adb b/gcc/ada/sem_ch5.adb
--- a/gcc/ada/sem_ch5.adb
+++ b/gcc/ada/sem_ch5.adb
@@ -3377,6 +3377,32 @@ package body Sem_Ch5 is
                        ("\loop executes zero times or raises "
                         & "Constraint_Error??", Bad_Bound);
                   end if;
+
+                  if Compile_Time_Compare (LLo, LHi, Assume_Valid => False)
+                    = GT
+                  then
+                     Error_Msg_N ("??constrained range is null",
+                       Constraint (DS));
+
+                     --  Additional constraints on modular types can be
+                     --  confusing, add more information.
+
+                     if Ekind (Etype (DS)) = E_Modular_Integer_Subtype then
+                        Error_Msg_Uint_1 := Intval (LLo);
+                        Error_Msg_Uint_2 := Intval (LHi);
+                        Error_Msg_NE ("\iterator has modular type &, " &
+                          "so the loop has bounds ^ ..^",
+                          Constraint (DS),
+                          Subtype_Mark (DS));
+                     end if;
+
+                     Set_Is_Null_Loop (Loop_Nod);
+                     Null_Range := True;
+
+                     --  Suppress other warnigns about the body of the loop, as
+                     --  it will never execute.
+                     Set_Suppress_Loop_Warnings (Loop_Nod);
+                  end if;
                end;
             end if;