2013-03-06  Tilo Schwarz  <tilo@tilo-schwarz.de>

	PR libfortran/48618
	* io/open.c (st_open): Raise error for unit number < 0 only if
	unit number does not exist already.
	* gcc/testsuite/gfortran.dg/open_negative_unit_1.f90: New.

diff --git a/gcc/testsuite/gfortran.dg/open_negative_unit_1.f90 b/gcc/testsuite/gfortran.dg/open_negative_unit_1.f90
new file mode 100644
index 0000000..6446436
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/open_negative_unit_1.f90
@@ -0,0 +1,21 @@
+! { dg-do run }
+! PR48618 - Negative unit number in OPEN(...) is sometimes allowed
+!
+! Test originally from Janne Blomqvist in PR:
+! http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48618
+
+program nutest
+    implicit none
+    integer id, ios
+
+    open(newunit=id, file="foo.txt", iostat=ios)
+    if (ios /= 0) call abort
+
+    open(id, file="bar.txt", iostat=ios)
+    if (ios /= 0) call abort
+
+    close(id, status="delete")
+
+    open(-10, file="foo.txt", iostat=ios)
+    if (ios == 0) call abort
+end program nutest
diff --git a/libgfortran/io/open.c b/libgfortran/io/open.c
index d9cfde8..19fab1d 100644
--- a/libgfortran/io/open.c
+++ b/libgfortran/io/open.c
@@ -818,10 +818,6 @@ st_open (st_parameter_open *opp)
 
   flags.convert = conv;
 
-  if (!(opp->common.flags & IOPARM_OPEN_HAS_NEWUNIT) && opp->common.unit < 0)
-    generate_error (&opp->common, LIBERROR_BAD_OPTION,
-		    "Bad unit number in OPEN statement");
-
   if (flags.position != POSITION_UNSPECIFIED
       && flags.access == ACCESS_DIRECT)
     generate_error (&opp->common, LIBERROR_BAD_OPTION,
@@ -848,8 +844,16 @@ st_open (st_parameter_open *opp)
     {
       if ((opp->common.flags & IOPARM_OPEN_HAS_NEWUNIT))
 	opp->common.unit = get_unique_unit_number(opp);
+      else if (opp->common.unit < 0)
+	{
+	  u = find_unit (opp->common.unit);
+	  if (u == NULL) /* Negative unit and no NEWUNIT-created unit found.  */
+	    generate_error (&opp->common, LIBERROR_BAD_OPTION,
+			    "Bad unit number in OPEN statement");
+	}
 
-      u = find_or_create_unit (opp->common.unit);
+      if (u == NULL)
+	u = find_or_create_unit (opp->common.unit);
       if (u->s == NULL)
 	{
 	  u = new_unit (opp, u, &flags);