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/ice-9/boot-9.scm

@@ -3665,7 +3665,8 @@ but it fails to load."
 
 (define %auto-compilation-options
   ;; Default `compile-file' option when auto-compiling.
-  '(#:warnings (unbound-variable macro-use-before-definition arity-mismatch
+  '(#:warnings (unbound-variable shadowed-toplevel
+                macro-use-before-definition arity-mismatch
                 format duplicate-case-datum bad-case-datum)))
 
 (define* (load-in-vicinity dir file-name #:optional reader)

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

@@ -2319,6 +2319,7 @@ 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)

module/system/base/message.scm

@@ -1,6 +1,6 @@
 ;;; User interface messages
 
-;; Copyright (C) 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+;; Copyright (C) 2009, 2010, 2011, 2012, 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
@@ -109,6 +109,13 @@
                (emit port "~A: warning: possibly unused local top-level variable `~A'~%"
                      loc name)))
 
+           (shadowed-toplevel
+            "report shadowed top-level variables"
+            ,(lambda (port loc name previous-loc)
+               (emit port "~A: warning: shadows previous definition of `~A' at ~A~%"
+                     loc name
+                     (location-string previous-loc))))
+
            (unbound-variable
             "report possibly unbound variables"
             ,(lambda (port loc name)