Add -Wshadow=global -Wshadow=local and -Wshadow=compatible-local.

This patch from Le-Chun Wu adds two new shadow warning flags for
C and C++:

  -Wshadow=local which warns if a local variable shadows another local
  variable or parameter,

  -Wshadow=compatible-local which warns if a local variable shadows
  another local variable or parameter whose type is compatible with
  that of the shadowing variable.

It is already on the google/main branch (Google ref 39127) and was
previously submitted by Diego Novillo and reviewed on
http://codereview.appspot.com/4452058

I addressed the review comments and made the following changes:
- Add -Wshadow=global (the default alias for -Wshadow).
- Make the documented options -Wshadow=global, -Wshadow=local
  and -Wshadow=compatible-local (with hidden undocumented aliases
  -Wshadow-local and -Wshadow-compatible-local for compatibility).
- The -Wshadow=global, -Wshadow=local and -Wshadow=compatible-local
  relationships are expressed in common.opt instead of in opts.c
  and documented in invoke.texi.
- The "previous declaration" warnings were turned into notes and use
  the (now) existing infrastructure instead of duplicating the warnings.
  The testcases have been adjusted to expect the notes.
- The conditional change in name-lookup.c for non-locals (where we
  don't want to change the warnings, but just check the global ones)
  has been dropped.
- Use warning_at in c-decl.c (warn_if_shadowing).

gcc/ChangeLog:
2016-10-30  Le-Chun Wu  <lcwu@google.com>
            Mark Wielaard  <mjw@redhat.com>

       * doc/invoke.texi: Document Wshadow-local and Wshadow-compatible-local.
       * common.opt (Wshadow=global): New option. Default for -Wshadow.
       (Wshadow=local): New option.
       (Wshadow-local): Hidden alias for -Wshadow=local.
       (Wshadow=compatible-local): New option.
       (Wshadow-compatible-local): Hidden alias for
       -Wshadow=compatible-local.
       * doc/invoke.texi: Document Wshadow=global, Wshadow=local and
       Wshadow=compatible-local.

gcc/c/ChangeLog:
2016-10-30  Le-Chun Wu  <lcwu@google.com>
            Mark Wielaard  <mjw@redhat.com>

       * c-decl.c (warn_if_shadowing): Use the warning code corresponding
       to the given -Wshadow= variant. Use warning_at.

gcc/cp/ChangeLog:
2016-10-30  Le-Chun Wu  <lcwu@google.com>
            Mark Wielaard  <mjw@redhat.com>

       * name-lookup.c (pushdecl_maybe_friend): When emitting a
       shadowing warning, use the code corresponding to the
       given -Wshadow= variant.

gcc/testsuite/ChangeLog
2016-10-30  Le-Chun Wu  <lcwu@google.com>
            Mark Wielaard  <mjw@redhat.com>

       * gcc.dg/Wshadow-compatible-local-1.c: New test.
       * gcc.dg/Wshadow-local-1.c: Likewise.
       * gcc.dg/Wshadow-local-2.c: Likewise.
       * g++.dg/warn/Wshadow-compatible-local-1.C: Likewise.
       * g++.dg/warn/Wshadow-local-1.C: Likewise.
       * g++.dg/warn/Wshadow-local-2.C: Likewise.

Co-Authored-By: Mark Wielaard <mjw@redhat.com>

From-SVN: r241699

1 files changed, 37 insertions, 6 deletions

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 136f304..3e1b7a4 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -2735,7 +2735,9 @@ warn_if_shadowing (tree new_decl)
   struct c_binding *b;
 
   /* Shadow warnings wanted?  */
-  if (!warn_shadow
+  if (!(warn_shadow
+        || warn_shadow_local
+        || warn_shadow_compatible_local)
       /* No shadow warnings for internally generated vars.  */
       || DECL_IS_BUILTIN (new_decl)
       /* No shadow warnings for vars made for inlining.  */
@@ -2759,9 +2761,23 @@ warn_if_shadowing (tree new_decl)
 	    break;
 	  }
 	else if (TREE_CODE (old_decl) == PARM_DECL)
-	  warned = warning (OPT_Wshadow,
-			    "declaration of %q+D shadows a parameter",
-			    new_decl);
+	  {
+	    enum opt_code warning_code;
+
+	    /* If '-Wshadow=compatible-local' is specified without other
+	       -Wshadow= flags, we will warn only when the types of the
+	       shadowing variable (i.e. new_decl) and the shadowed variable
+	       (old_decl) are compatible.  */
+	    if (warn_shadow)
+	      warning_code = OPT_Wshadow;
+	    else if (comptypes (TREE_TYPE (old_decl), TREE_TYPE (new_decl)))
+	      warning_code = OPT_Wshadow_compatible_local;
+	    else
+	      warning_code = OPT_Wshadow_local;
+	    warned = warning_at (DECL_SOURCE_LOCATION (new_decl), warning_code,
+				 "declaration of %qD shadows a parameter",
+				 new_decl);
+	  }
 	else if (DECL_FILE_SCOPE_P (old_decl))
 	  {
 	    /* Do not warn if a variable shadows a function, unless
@@ -2784,8 +2800,23 @@ warn_if_shadowing (tree new_decl)
 	    break;
 	  }
 	else
-	  warned = warning (OPT_Wshadow, "declaration of %q+D shadows a "
-			    "previous local", new_decl);
+	  {
+	    enum opt_code warning_code;
+
+	    /* If '-Wshadow=compatible-local' is specified without other
+	       -Wshadow= flags, we will warn only when the types of the
+	       shadowing variable (i.e. new_decl) and the shadowed variable
+	       (old_decl) are compatible.  */
+	    if (warn_shadow)
+	      warning_code = OPT_Wshadow;
+	    else if (comptypes (TREE_TYPE (old_decl), TREE_TYPE (new_decl)))
+	      warning_code = OPT_Wshadow_compatible_local;
+	    else
+	      warning_code = OPT_Wshadow_local;
+	    warned = warning_at (DECL_SOURCE_LOCATION (new_decl), warning_code,
+				 "declaration of %qD shadows a previous local",
+				 new_decl);
+	  }
 
 	if (warned)
 	  inform (DECL_SOURCE_LOCATION (old_decl),