[Ada] Fix proof of runtime unit a-strfix and a-strsup

[Pierre-Marie de Rodat via Gcc-patches]

Update to provers caused some proof regressions.  Fix the proof by
adding an assertion.

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

gcc/ada/

        * libgnat/a-strfix.adb: Add assertions.
        * libgnat/a-strsup.adb: Idem.
diff --git a/gcc/ada/libgnat/a-strfix.adb b/gcc/ada/libgnat/a-strfix.adb
--- a/gcc/ada/libgnat/a-strfix.adb
+++ b/gcc/ada/libgnat/a-strfix.adb
@@ -628,6 +628,11 @@ package body Ada.Strings.Fixed with SPARK_Mode is
                  (Result (1 .. Integer'Max (0, Low - Source'First))
                   = Source (Source'First .. Low - 1));
                Result (Front_Len + 1 .. Front_Len + By'Length) := By;
+               pragma Assert
+                 (Result
+                    (Integer'Max (0, Low - Source'First) + 1
+                     .. Integer'Max (0, Low - Source'First) + By'Length)
+                  = By);
 
                if High < Source'Last then
                   Result (Front_Len + By'Length + 1 .. Result'Last) :=


diff --git a/gcc/ada/libgnat/a-strsup.adb b/gcc/ada/libgnat/a-strsup.adb
--- a/gcc/ada/libgnat/a-strsup.adb
+++ b/gcc/ada/libgnat/a-strsup.adb
@@ -1150,6 +1150,14 @@ package body Ada.Strings.Superbounded with SPARK_Mode is
          Result.Data (Position .. Position - 1 + New_Item'Length) :=
            Super_String_Data (New_Item);
          Result.Current_Length := Source.Current_Length;
+         pragma Assert
+           (String'(Super_Slice (Result, 1, Position - 1)) =
+              Super_Slice (Source, 1, Position - 1));
+         pragma Assert
+           (Super_Slice (Result,
+            Position, Position - 1 + New_Item'Length) =
+              New_Item);
+
          return Result;
 
       elsif Position - 1 <= Max_Length - New_Item'Length then
@@ -1157,6 +1165,14 @@ package body Ada.Strings.Superbounded with SPARK_Mode is
          Result.Data (Position .. Position - 1 + New_Item'Length) :=
            Super_String_Data (New_Item);
          Result.Current_Length := Position - 1 + New_Item'Length;
+         pragma Assert
+           (String'(Super_Slice (Result, 1, Position - 1)) =
+              Super_Slice (Source, 1, Position - 1));
+         pragma Assert
+           (Super_Slice (Result,
+            Position, Position - 1 + New_Item'Length) =
+              New_Item);
+
          return Result;
 
       else
@@ -1189,6 +1205,7 @@ package body Ada.Strings.Superbounded with SPARK_Mode is
          end case;
 
          Result.Current_Length := Max_Length;
+         pragma Assert (Super_Length (Result) = Source.Max_Length);
          return Result;
       end if;
    end Super_Overwrite;