C/C++ PATCH to add -Wdangling-else option

[Marek Polacek <polacek at redhat dot com>]

This patch is meant to be applied on top of the "Wparentheses overhaul" patch.

I really think that warning about the dangling else problem isn't appropriate
as a part of the -Wparentheses warning, which I think should only deal with
stuff like precedence of operators, i.e. things where ()'s are missing and not
{}'s.

This new warning is, however, a subset of -Wparentheses.

Bootstrapped/regtested on x86_64-linux, ok for trunk or should I stash it
for the next stage1?

2016-04-13  Marek Polacek  <polacek@redhat.com>

	* c.opt (Wdangling-else): New option.

	* c-parser.c (c_parser_if_statement): Replace OPT_Wparentheses with
	OPT_Wdangling_else.

	* parser.c (cp_parser_selection_statement): Replace OPT_Wparentheses
	with OPT_Wdangling_else.

	* doc/invoke.texi: Document -Wdangling-else.

	* c-c++-common/Wdangling-else-1.c: New test.
	* c-c++-common/Wdangling-else-2.c: New test.
	* c-c++-common/Wdangling-else-3.c: New test.

diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 4f86876..5e7bcb8 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -370,6 +370,10 @@ Wctor-dtor-privacy
 C++ ObjC++ Var(warn_ctor_dtor_privacy) Warning
 Warn when all constructors and destructors are private.
 
+Wdangling-else
+C ObjC C++ ObjC++ Var(warn_dangling_else) Warning LangEnabledBy(C ObjC C++ ObjC++,Wparentheses)
+Warn about dangling else.
+
 Wdate-time
 C ObjC C++ ObjC++ CPP(warn_date_time) CppReason(CPP_W_DATE_TIME) Var(cpp_warn_date_time) Init(0) Warning
 Warn about __TIME__, __DATE__ and __TIMESTAMP__ usage.
diff --git gcc/c/c-parser.c gcc/c/c-parser.c
index cafcb99..1f95446 100644
--- gcc/c/c-parser.c
+++ gcc/c/c-parser.c
@@ -5486,7 +5486,7 @@ c_parser_if_statement (c_parser *parser, bool *if_p, vec<tree> *chain)
       /* Diagnose an ambiguous else if if-then-else is nested inside
 	 if-then.  */
       if (nested_if)
-	warning_at (loc, OPT_Wparentheses,
+	warning_at (loc, OPT_Wdangling_else,
 		    "suggest explicit braces to avoid ambiguous %<else%>");
 
       if (warn_duplicated_cond)
diff --git gcc/cp/parser.c gcc/cp/parser.c
index 00e211e..968706f 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -10951,7 +10951,7 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 		   statement which does have an else clause.  We warn
 		   about the potential ambiguity.  */
 		if (nested_if)
-		  warning_at (EXPR_LOCATION (statement), OPT_Wparentheses,
+		  warning_at (EXPR_LOCATION (statement), OPT_Wdangling_else,
 			      "suggest explicit braces to avoid ambiguous"
 			      " %<else%>");
 		if (warn_duplicated_cond)
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index e9763d4..6253d78 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -257,7 +257,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wc90-c99-compat -Wc99-c11-compat @gol
 -Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align  -Wcast-qual  @gol
 -Wchar-subscripts -Wclobbered  -Wcomment -Wconditionally-supported  @gol
--Wconversion -Wcoverage-mismatch -Wno-cpp -Wdate-time -Wdelete-incomplete @gol
+-Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time @gol
+-Wdelete-incomplete @gol
 -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init @gol
 -Wdisabled-optimization @gol
 -Wno-discarded-qualifiers -Wno-discarded-array-qualifiers @gol
@@ -3974,46 +3975,6 @@ Also warn if a comparison like @code{x<=y<=z} appears; this is
 equivalent to @code{(x<=y ? 1 : 0) <= z}, which is a different
 interpretation from that of ordinary mathematical notation.
 
-Also warn about constructions where there may be confusion to which
-@code{if} statement an @code{else} branch belongs.  Here is an example of
-such a case:
-
-@smallexample
-@group
-@{
-  if (a)
-    if (b)
-      foo ();
-  else
-    bar ();
-@}
-@end group
-@end smallexample
-
-In C/C++, every @code{else} branch belongs to the innermost possible
-@code{if} statement, which in this example is @code{if (b)}.  This is
-often not what the programmer expected, as illustrated in the above
-example by indentation the programmer chose.  When there is the
-potential for this confusion, GCC issues a warning when this flag
-is specified.  To eliminate the warning, add explicit braces around
-the innermost @code{if} statement so there is no way the @code{else}
-can belong to the enclosing @code{if}.  The resulting code
-looks like this:
-
-@smallexample
-@group
-@{
-  if (a)
-    @{
-      if (b)
-        foo ();
-      else
-        bar ();
-    @}
-@}
-@end group
-@end smallexample
-
 Also warn for dangerous uses of the GNU extension to
 @code{?:} with omitted middle operand. When the condition
 in the @code{?}: operator is a boolean expression, the omitted value is
@@ -5146,6 +5107,51 @@ compiler doesn't give this warning for types defined in the main .C
 file, as those are unlikely to have multiple definitions.
 @option{-Wsubobject-linkage} is enabled by default.
 
+@item -Wdangling-else
+@opindex Wdangling-else
+@opindex Wno-dangling-else
+Warn about constructions where there may be confusion to which
+@code{if} statement an @code{else} branch belongs.  Here is an example of
+such a case:
+
+@smallexample
+@group
+@{
+  if (a)
+    if (b)
+      foo ();
+  else
+    bar ();
+@}
+@end group
+@end smallexample
+
+In C/C++, every @code{else} branch belongs to the innermost possible
+@code{if} statement, which in this example is @code{if (b)}.  This is
+often not what the programmer expected, as illustrated in the above
+example by indentation the programmer chose.  When there is the
+potential for this confusion, GCC issues a warning when this flag
+is specified.  To eliminate the warning, add explicit braces around
+the innermost @code{if} statement so there is no way the @code{else}
+can belong to the enclosing @code{if}.  The resulting code
+looks like this:
+
+@smallexample
+@group
+@{
+  if (a)
+    @{
+      if (b)
+        foo ();
+      else
+        bar ();
+    @}
+@}
+@end group
+@end smallexample
+
+This warning is enabled by @option{-Wparentheses}.
+
 @item -Wdate-time
 @opindex Wdate-time
 @opindex Wno-date-time
diff --git gcc/testsuite/c-c++-common/Wdangling-else-1.c gcc/testsuite/c-c++-common/Wdangling-else-1.c
index e69de29..28a5a8f 100644
--- gcc/testsuite/c-c++-common/Wdangling-else-1.c
+++ gcc/testsuite/c-c++-common/Wdangling-else-1.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-Wdangling-else" } */
+
+void bar (int);
+void
+foo (int a, int b)
+{
+  if (a) /* { dg-warning "suggest explicit braces to avoid ambiguous" } */
+    if (b)
+      bar (1);
+  else
+    bar (2);
+}
diff --git gcc/testsuite/c-c++-common/Wdangling-else-2.c gcc/testsuite/c-c++-common/Wdangling-else-2.c
index e69de29..87ea1ab 100644
--- gcc/testsuite/c-c++-common/Wdangling-else-2.c
+++ gcc/testsuite/c-c++-common/Wdangling-else-2.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-Wparentheses" } */
+
+void bar (int);
+void
+foo (int a, int b)
+{
+  if (a) /* { dg-warning "suggest explicit braces to avoid ambiguous" } */
+    if (b)
+      bar (1);
+  else
+    bar (2);
+}
diff --git gcc/testsuite/c-c++-common/Wdangling-else-3.c gcc/testsuite/c-c++-common/Wdangling-else-3.c
index e69de29..0dae0d5 100644
--- gcc/testsuite/c-c++-common/Wdangling-else-3.c
+++ gcc/testsuite/c-c++-common/Wdangling-else-3.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-Wparentheses -Wno-dangling-else" } */
+
+void bar (int);
+void
+foo (int a, int b)
+{
+  if (a) /* { dg-bogus "suggest explicit braces to avoid ambiguous" } */
+    if (b)
+      bar (1);
+  else
+    bar (2);
+}

	Marek