Index: varasm.c
===================================================================
--- varasm.c    (revision 185762)
+++ varasm.c    (working copy)
@@ -2166,12 +2166,18 @@ static GTY(()) tree weak_decls;
 void
 assemble_external (tree decl ATTRIBUTE_UNUSED)
 {
-  /* Because most platforms do not define ASM_OUTPUT_EXTERNAL, the
-     main body of this code is only rarely exercised.  To provide some
-     testing, on all platforms, we make sure that the ASM_OUT_FILE is
-     open.  If it's not, we should not be calling this function.  */
+  /*  Make sure that the ASM_OUT_FILE is open.
+      If it's not, we should not be calling this function.  */
  gcc_assert (asm_out_file);

+  /* This function should only be called if we are expanding, or have
+     expanded, to RTL.
+     Ideally, only final.c would be calling this function, but it is
+     not clear whether that would break things somehow.  See PR 17982
+     for further discussion.  */
+  gcc_assert (cgraph_state == CGRAPH_STATE_EXPANSION
+              || cgraph_state == CGRAPH_STATE_FINISHED);
+
  if (!DECL_P (decl) || !DECL_EXTERNAL (decl) || !TREE_PUBLIC (decl))
    return;