Add -Wshadowed-toplevel.

* module/language/tree-il/analyze.scm (shadowed-toplevel-analysis): New
variable.
* module/language/tree-il/compile-cps.scm (%warning-passes): Add it.
* module/system/base/message.scm (%warning-types): Add it.
* test-suite/tests/tree-il.test ("warnings")["shadowed-toplevel"]: New
test prefix.
* module/ice-9/boot-9.scm (%auto-compilation-options): Add it.
* doc/ref/api-evaluation.texi (Compilation): Add 'shadowed-toplevel' and
'macro-use-before-definition'.

module/language/tree-il/analyze.scm

@@ -1,6 +1,6 @@
 ;;; TREE-IL -> GLIL compiler
 
-;; Copyright (C) 2001, 2008-2014 Free Software Foundation, Inc.
+;; Copyright (C) 2001, 2008-2014, 2018 Free Software Foundation, Inc.
 
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -34,6 +34,7 @@
             analyze-tree
             unused-variable-analysis
             unused-toplevel-analysis
+            shadowed-toplevel-analysis
             unbound-variable-analysis
             macro-use-before-definition-analysis
             arity-analysis
@@ -813,6 +814,37 @@ given `tree-il' element."
 
      (make-reference-graph vlist-null vlist-null #f))))
 
+
+;;;
+;;; Shadowed top-level definition analysis.
+;;;
+
+(define shadowed-toplevel-analysis
+  ;; Report top-level definitions that shadow previous top-level
+  ;; definitions from the same compilation unit.
+  (make-tree-analysis
+   (lambda (x defs env locs)
+     ;; Going down into X.
+     (record-case x
+                  ((<toplevel-define> name src)
+                   (match (vhash-assq name defs)
+                     ((_ . previous-definition)
+                      (warning 'shadowed-toplevel src name
+                               (toplevel-define-src previous-definition))
+                      defs)
+                     (#f
+                      (vhash-consq name x defs))))
+                  (else defs)))
+
+   (lambda (x defs env locs)
+     ;; Leaving X's scope.
+     defs)
+
+   (lambda (defs env)
+     #t)
+
+   vlist-null))
+
 
 ;;;
 ;;; Unbound variable analysis.

module/language/tree-il/compile-cps.scm

@@ -1,6 +1,6 @@
 ;;; Continuation-passing style (CPS) intermediate language (IL)
 
-;; Copyright (C) 2013, 2014, 2015, 2017 Free Software Foundation, Inc.
+;; Copyright (C) 2013, 2014, 2015, 2017, 2018 Free Software Foundation, Inc.
 
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -1014,6 +1014,7 @@ integer."
 (define %warning-passes
   `((unused-variable             . ,unused-variable-analysis)
     (unused-toplevel             . ,unused-toplevel-analysis)
+    (shadowed-toplevel           . ,shadowed-toplevel-analysis)
     (unbound-variable            . ,unbound-variable-analysis)
     (macro-use-before-definition . ,macro-use-before-definition-analysis)
     (arity-mismatch              . ,arity-analysis)