Hi,
this is what I did to add the warning requested in the PR. Among the
slightly less trivial bits:
1- In order not to risk warning *before* an hard error is produced by
build_*_cast_1 code, I'm warning only if the cast is actually
successful, thus at the end of each build_*_cast function.
2- In order not to warn when the useless cast shows up upon template
instantiation I'm using here too the c_inhibit_evaluation_warnings trick
(as we do already for warn_sign_compare, I believe).
3- References can be easily missed because wrapped in INDIRECT_REF: as
explained at the beginning of tree.c and already used in many places, a
REFERENCE_REF_P check (and in case a TREE_OPERAND (expr, 0)) takes care
of that. I'm not 100% sure the solution is fully general, though.
4- I'm naming the function implementing the warning itself
maybe_warn_about_useless_cast, which seems consistent with the name of
at least another warning in typeck.c but other options are possible, of
course...
Booted and tested x86_64-linux.
Thanks,
Paolo.
//////////////////////
2012-03-16 Paolo Carlini <paolo.carl...@oracle.com>
PR c++/14710
* doc/invoke.texi: Document -Wuseless-cast.
/c-family
2012-03-16 Paolo Carlini <paolo.carl...@oracle.com>
PR c++/14710
* c.opt ([Wuseless-cast]): Add.
/cp
2012-03-16 Paolo Carlini <paolo.carl...@oracle.com>
PR c++/14710
* cp-tree.h (maybe_warn_about_useless_cast): Declare.
* typeck.c (maybe_warn_about_useless_cast): Define.
(build_reinterpret_cast, build_const_cast,
build_static_cast, cp_build_c_cast): Use it.
* rtti.c (build_dynamic_cast): Likewise.
* pt.c (tsubst_copy_and_build, case CAST_EXPR): Increment/decrement
c_inhibit_evaluation_warnings before/after the build_* calls.
/testsuite
2012-03-16 Paolo Carlini <paolo.carl...@oracle.com>
PR c++/14710
* g++.dg/warn/Wuseless-cast.C: New.
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi (revision 185462)
+++ doc/invoke.texi (working copy)
@@ -274,8 +274,8 @@ Objective-C and Objective-C++ Dialects}.
-Wunused-label -Wunused-local-typedefs -Wunused-parameter @gol
-Wno-unused-result -Wunused-value @gol -Wunused-variable @gol
-Wunused-but-set-parameter -Wunused-but-set-variable @gol
--Wvariadic-macros -Wvector-operation-performance -Wvla
--Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant}
+-Wuseless-cast -Wvariadic-macros -Wvector-operation-performance @gol
+-Wvla -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant}
@item C and Objective-C-only Warning Options
@gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
@@ -4199,6 +4199,11 @@ types. @option{-Wconversion-null} is enabled by de
Warn when a literal '0' is used as null pointer constant. This can
be useful to facilitate the conversion to @code{nullptr} in C++11.
+@item -Wuseless-cast @r{(C++ and Objective-C++ only)}
+@opindex Wuseless-cast
+@opindex Wno-useless-cast
+Warn when an expression is casted to its own type.
+
@item -Wempty-body
@opindex Wempty-body
@opindex Wno-empty-body
Index: c-family/c.opt
===================================================================
--- c-family/c.opt (revision 185462)
+++ c-family/c.opt (working copy)
@@ -697,6 +697,10 @@ Wzero-as-null-pointer-constant
C++ ObjC++ Var(warn_zero_as_null_pointer_constant) Warning
Warn when a literal '0' is used as null pointer
+Wuseless-cast
+C++ ObjC++ Var(warn_useless_cast) Warning
+Warn about useless casts
+
ansi
C ObjC C++ ObjC++
A synonym for -std=c89 (for C) or -std=c++98 (for C++)
Index: testsuite/g++.dg/warn/Wuseless-cast.C
===================================================================
--- testsuite/g++.dg/warn/Wuseless-cast.C (revision 0)
+++ testsuite/g++.dg/warn/Wuseless-cast.C (revision 0)
@@ -0,0 +1,62 @@
+// { dg-options "-Wuseless-cast" }
+
+template<typename T>
+ void tmpl_f1(T t)
+ {
+ (int)(t);
+ static_cast<int>(t);
+ reinterpret_cast<int>(t);
+ }
+
+struct A { };
+
+template<typename T>
+ void tmpl_f2(T& t)
+ {
+ (A*)(&t);
+ const_cast<A*>(&t);
+ static_cast<A*>(&t);
+ reinterpret_cast<A*>(&t);
+ dynamic_cast<A*>(&t);
+ }
+
+template<typename T>
+ void tmpl_f3(T& t)
+ {
+ (A&)(t);
+ const_cast<A&>(t);
+ static_cast<A&>(t);
+ reinterpret_cast<A&>(t);
+ dynamic_cast<A&>(t);
+ }
+
+void f()
+{
+ int n;
+
+ (int)(n); // { dg-warning "useless cast" }
+ static_cast<int>(n); // { dg-warning "useless cast" }
+ reinterpret_cast<int>(n); // { dg-warning "useless cast" }
+
+ tmpl_f1(n);
+
+ A a;
+
+ (A*)(&a); // { dg-warning "useless cast" }
+ const_cast<A*>(&a); // { dg-warning "useless cast" }
+ static_cast<A*>(&a); // { dg-warning "useless cast" }
+ reinterpret_cast<A*>(&a); // { dg-warning "useless cast" }
+ dynamic_cast<A*>(&a); // { dg-warning "useless cast" }
+
+ tmpl_f2(a);
+
+ A& b = a;
+
+ (A&)(b); // { dg-warning "useless cast" }
+ const_cast<A&>(b); // { dg-warning "useless cast" }
+ static_cast<A&>(b); // { dg-warning "useless cast" }
+ reinterpret_cast<A&>(b); // { dg-warning "useless cast" }
+ dynamic_cast<A&>(b); // { dg-warning "useless cast" }
+
+ tmpl_f3(b);
+}
Index: cp/typeck.c
===================================================================
--- cp/typeck.c (revision 185473)
+++ cp/typeck.c (working copy)
@@ -5749,6 +5749,24 @@ check_for_casting_away_constness (tree src_type, t
}
}
+/*
+ Warns if the cast from expression EXPR to type TYPE is useless.
+ */
+void
+maybe_warn_about_useless_cast (tree type, tree expr, tsubst_flags_t complain)
+{
+ if (complain & tf_warning
+ && c_inhibit_evaluation_warnings == 0)
+ {
+ if (REFERENCE_REF_P (expr))
+ expr = TREE_OPERAND (expr, 0);
+
+ if (same_type_p (TREE_TYPE (expr), type))
+ warning (OPT_Wuseless_cast, "useless cast from type %qT to itself",
+ type);
+ }
+}
+
/* Convert EXPR (an expression with pointer-to-member type) to TYPE
(another pointer-to-member type in the same hierarchy) and return
the converted expression. If ALLOW_INVERSE_P is permitted, a
@@ -6078,7 +6096,11 @@ build_static_cast (tree type, tree expr, tsubst_fl
result = build_static_cast_1 (type, expr, /*c_cast_p=*/false, &valid_p,
complain);
if (valid_p)
- return result;
+ {
+ if (result != error_mark_node)
+ maybe_warn_about_useless_cast (type, expr, complain);
+ return result;
+ }
if (complain & tf_error)
error ("invalid static_cast from type %qT to type %qT",
@@ -6305,6 +6327,8 @@ build_reinterpret_cast_1 (tree type, tree expr, bo
tree
build_reinterpret_cast (tree type, tree expr, tsubst_flags_t complain)
{
+ tree r;
+
if (type == error_mark_node || expr == error_mark_node)
return error_mark_node;
@@ -6319,8 +6343,11 @@ build_reinterpret_cast (tree type, tree expr, tsub
return convert_from_reference (t);
}
- return build_reinterpret_cast_1 (type, expr, /*c_cast_p=*/false,
- /*valid_p=*/NULL, complain);
+ r = build_reinterpret_cast_1 (type, expr, /*c_cast_p=*/false,
+ /*valid_p=*/NULL, complain);
+ if (r != error_mark_node)
+ maybe_warn_about_useless_cast (type, expr, complain);
+ return r;
}
/* Perform a const_cast from EXPR to TYPE. If the cast is valid,
@@ -6464,6 +6491,8 @@ build_const_cast_1 (tree dst_type, tree expr, tsub
tree
build_const_cast (tree type, tree expr, tsubst_flags_t complain)
{
+ tree r;
+
if (type == error_mark_node || error_operand_p (expr))
return error_mark_node;
@@ -6478,8 +6507,10 @@ build_const_cast (tree type, tree expr, tsubst_fla
return convert_from_reference (t);
}
- return build_const_cast_1 (type, expr, complain,
- /*valid_p=*/NULL);
+ r = build_const_cast_1 (type, expr, complain, /*valid_p=*/NULL);
+ if (r != error_mark_node)
+ maybe_warn_about_useless_cast (type, expr, complain);
+ return r;
}
/* Like cp_build_c_cast, but for the c-common bits. */
@@ -6567,7 +6598,11 @@ cp_build_c_cast (tree type, tree expr, tsubst_flag
result = build_const_cast_1 (type, value, complain & tf_warning,
&valid_p);
if (valid_p)
- return result;
+ {
+ if (result != error_mark_node)
+ maybe_warn_about_useless_cast (type, value, complain);
+ return result;
+ }
/* Or a static cast. */
result = build_static_cast_1 (type, value, /*c_cast_p=*/true,
@@ -6580,11 +6615,13 @@ cp_build_c_cast (tree type, tree expr, tsubst_flag
const_cast. */
if (valid_p
/* A valid cast may result in errors if, for example, a
- conversion to am ambiguous base class is required. */
+ conversion to an ambiguous base class is required. */
&& !error_operand_p (result))
{
tree result_type;
+ maybe_warn_about_useless_cast (type, value, complain);
+
/* Non-class rvalues always have cv-unqualified type. */
if (!CLASS_TYPE_P (type))
type = TYPE_MAIN_VARIANT (type);
Index: cp/rtti.c
===================================================================
--- cp/rtti.c (revision 185473)
+++ cp/rtti.c (working copy)
@@ -1,6 +1,6 @@
/* RunTime Type Identification
Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004,
- 2005, 2006, 2007, 2008, 2009, 2010, 2011
+ 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012
Free Software Foundation, Inc.
Mostly written by Jason Merrill (ja...@cygnus.com).
@@ -774,6 +774,8 @@ build_dynamic_cast_1 (tree type, tree expr, tsubst
tree
build_dynamic_cast (tree type, tree expr, tsubst_flags_t complain)
{
+ tree r;
+
if (type == error_mark_node || expr == error_mark_node)
return error_mark_node;
@@ -784,9 +786,12 @@ build_dynamic_cast (tree type, tree expr, tsubst_f
return convert_from_reference (expr);
}
- return convert_from_reference (build_dynamic_cast_1 (type, expr, complain));
+ r = convert_from_reference (build_dynamic_cast_1 (type, expr, complain));
+ if (r != error_mark_node)
+ maybe_warn_about_useless_cast (type, expr, complain);
+ return r;
}
-
+
/* Return the runtime bit mask encoding the qualifiers of TYPE. */
static int
Index: cp/pt.c
===================================================================
--- cp/pt.c (revision 185473)
+++ cp/pt.c (working copy)
@@ -13444,7 +13444,7 @@ tsubst_copy_and_build (tree t,
case STATIC_CAST_EXPR:
{
tree type;
- tree op;
+ tree op, r = NULL_TREE;
type = tsubst (TREE_TYPE (t), args, complain, in_decl);
if (integral_constant_expression_p
@@ -13458,21 +13458,30 @@ tsubst_copy_and_build (tree t,
op = RECUR (TREE_OPERAND (t, 0));
+ ++c_inhibit_evaluation_warnings;
switch (TREE_CODE (t))
{
case CAST_EXPR:
- return build_functional_cast (type, op, complain);
+ r = build_functional_cast (type, op, complain);
+ break;
case REINTERPRET_CAST_EXPR:
- return build_reinterpret_cast (type, op, complain);
+ r = build_reinterpret_cast (type, op, complain);
+ break;
case CONST_CAST_EXPR:
- return build_const_cast (type, op, complain);
+ r = build_const_cast (type, op, complain);
+ break;
case DYNAMIC_CAST_EXPR:
- return build_dynamic_cast (type, op, complain);
+ r = build_dynamic_cast (type, op, complain);
+ break;
case STATIC_CAST_EXPR:
- return build_static_cast (type, op, complain);
+ r = build_static_cast (type, op, complain);
+ break;
default:
gcc_unreachable ();
}
+ --c_inhibit_evaluation_warnings;
+
+ return r;
}
case POSTDECREMENT_EXPR:
Index: cp/cp-tree.h
===================================================================
--- cp/cp-tree.h (revision 185473)
+++ cp/cp-tree.h (working copy)
@@ -5860,6 +5860,7 @@ extern int lvalue_or_else (tree, enum
lvalue_use
extern void check_template_keyword (tree);
extern bool check_raw_literal_operator (const_tree decl);
extern bool check_literal_operator_args (const_tree, bool *,
bool *);
+extern void maybe_warn_about_useless_cast (tree, tree, tsubst_flags_t);
/* in typeck2.c */
extern void require_complete_eh_spec_types (tree, tree);
