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/usr/src/gcc-5/debian/patches/ada-749574.diff is in gcc-5-source 5.5.0-12ubuntu1.

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From: Ludovic Brenta <lbrenta@debian.org>
Forwarded: no
Bug-Debian: http://bugs.debian.org/749574
Description: Constraint_Error, range check failed at gnatlink.adb:2195, when called from gnatmake with -D option
 The procedure gnatlink assumes that the Linker_Options.Table contains access
 values to strings whose 'First index is always 1.  This assumption is wrong
 for the string returned by function Base_Name.
.
 Instead of fixing the assumption in many places, this patch changes the
 function Base_Name always to return a string with 'First=1.
.
 This looks like an upstream bug but strangely the reporter of this bug
 says it does not happen on GCC built from upstream sources.  Further
 investigation is required to determine whether or not to forward this
 bug and patch upstream.

Index: b/src/gcc/ada/gnatlink.adb
===================================================================
--- a/src/gcc/ada/gnatlink.adb
+++ b/src/gcc/ada/gnatlink.adb
@@ -266,7 +266,12 @@ procedure Gnatlink is
          Findex2 := File_Name'Last + 1;
       end if;
 
-      return File_Name (Findex1 .. Findex2 - 1);
+      declare
+         Result : String (1 .. Findex2 - Findex1);
+      begin
+         Result (1 .. Findex2 - Findex1) := File_Name (Findex1 .. Findex2 - 1);
+         return Result;
+      end;
    end Base_Name;
 
    -------------------------------