diff --git a/gcc/fortran/frontend-passes.c b/gcc/fortran/frontend-passes.c
index cc718e98c8b6..c8d7322dc3ce 100644
--- a/gcc/fortran/frontend-passes.c
+++ b/gcc/fortran/frontend-passes.c
@@ -156,6 +156,13 @@ realloc_string_callback (gfc_code **c, int *walk_subtrees,
   if (!gfc_check_dependency (expr1, expr2, true))
     return 0;
 
+  /* gfc_check_dependency doesn't always pick up identical expressions.
+     However, eliminating the above sends the compiler into an infinite
+     loop on valid expressions.  Without this check, the gimplifier emits
+     an ICE for a = a, where a is deferred character length.  */
+  if (!gfc_dep_compare_expr (expr1, expr2))
+    return 0;
+
   current_code = c;
   inserted_block = NULL;
   changed_statement = NULL;
diff --git a/gcc/testsuite/gfortran.dg/pr70673.f90 b/gcc/testsuite/gfortran.dg/pr70673.f90
new file mode 100644
index 000000000000..67856e0332e9
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr70673.f90
@@ -0,0 +1,25 @@
+! { dg-do run }
+!
+! Test the fix for PR70673
+!
+! Contributed by David Kinniburgh  <davidgkinniburgh@yahoo.co.uk>
+!
+module m
+contains
+  subroutine s(inp)
+    character(*), intent(in) :: inp
+    character(:), allocatable :: a
+    a = a           ! This used to ICE.
+    a = inp
+    a = a           ! This used to ICE too
+    if ((len (a) .ne. 5) .or. (a .ne. "hello")) call abort
+    a = a(2:3)      ! Make sure that temporary creation is not broken.
+    if ((len (a) .ne. 2) .or. (a .ne. "el")) call abort
+    deallocate (a)
+    a = a           ! This would ICE too.
+  end subroutine s
+end module m
+
+  use m
+  call s("hello")
+end