* [PATCH] c++: Disable -Wdangling-reference when initing T&
@ 2022-11-11 20:22 Marek Polacek
2022-11-14 23:16 ` Jason Merrill
0 siblings, 1 reply; 2+ messages in thread
From: Marek Polacek @ 2022-11-11 20:22 UTC (permalink / raw)
To: Jason Merrill, GCC Patches
Non-const lvalue references can't bind to a temporary, so the
warning should not be emitted if we're initializing something of that
type. I'm not disabling the warning when the function itself returns
a non-const lvalue reference, that would regress at least
const int &r = std::any_cast<int&>(std::any());
in Wdangling-reference2.C where the any_cast returns an int&.
Unfortunately, this patch means we'll stop diagnosing
int& fn(int&& x) { return static_cast<int&>(x); }
void test ()
{
int &r = fn(4);
}
where there's a genuine dangling reference. OTOH, the patch
should suppress false positives with iterators, like:
auto &candidate = *candidates.begin ();
and arguably that's more important than detecting some relatively
obscure cases. It's probably not worth it making the warning more
complicated by, for instance, not warning when a fn returns 'int&'
but takes 'const int&' (because then it can't return its argument).
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
gcc/cp/ChangeLog:
* call.cc (maybe_warn_dangling_reference): Don't warn when initializing
a non-const lvalue reference.
gcc/testsuite/ChangeLog:
* g++.dg/cpp23/elision4.C: Remove dg-warning.
* g++.dg/warn/Wdangling-reference1.C: Turn dg-warning into dg-bogus.
* g++.dg/warn/Wdangling-reference7.C: New test.
---
gcc/cp/call.cc | 10 ++++++++--
gcc/testsuite/g++.dg/cpp23/elision4.C | 4 ++--
gcc/testsuite/g++.dg/warn/Wdangling-reference1.C | 4 ++--
gcc/testsuite/g++.dg/warn/Wdangling-reference7.C | 16 ++++++++++++++++
4 files changed, 28 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index bd3b64a7e26..ef618d5c485 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13679,8 +13679,14 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
{
if (!warn_dangling_reference)
return;
- if (!(TYPE_REF_OBJ_P (TREE_TYPE (decl))
- || std_pair_ref_ref_p (TREE_TYPE (decl))))
+ tree type = TREE_TYPE (decl);
+ /* Only warn if what we're initializing has type T&& or const T&, or
+ std::pair<const T&, const T&>. (A non-const lvalue reference can't
+ bind to a temporary.) */
+ if (!((TYPE_REF_OBJ_P (type)
+ && (TYPE_REF_IS_RVALUE (type)
+ || CP_TYPE_CONST_P (TREE_TYPE (type))))
+ || std_pair_ref_ref_p (type)))
return;
/* Don't suppress the diagnostic just because the call comes from
a system header. If the DECL is not in a system header, or if
diff --git a/gcc/testsuite/g++.dg/cpp23/elision4.C b/gcc/testsuite/g++.dg/cpp23/elision4.C
index d39053ad741..77dcffcdaad 100644
--- a/gcc/testsuite/g++.dg/cpp23/elision4.C
+++ b/gcc/testsuite/g++.dg/cpp23/elision4.C
@@ -34,6 +34,6 @@ T& temporary2(T&& x) { return static_cast<T&>(x); }
void
test ()
{
- int& r1 = temporary1 (42); // { dg-warning "dangling reference" }
- int& r2 = temporary2 (42); // { dg-warning "dangling reference" }
+ int& r1 = temporary1 (42);
+ int& r2 = temporary2 (42);
}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
index 97c81ee716c..1718c28165e 100644
--- a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
@@ -139,6 +139,6 @@ struct Y {
// x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
int&& x1 = Y(); // { dg-warning "dangling reference" }
int&& x2 = Y{}; // { dg-warning "dangling reference" }
-int& x3 = Y(); // { dg-warning "dangling reference" }
-int& x4 = Y{}; // { dg-warning "dangling reference" }
+int& x3 = Y(); // { dg-bogus "dangling reference" }
+int& x4 = Y{}; // { dg-bogus "dangling reference" }
const int& t1 = Y().foo(10); // { dg-warning "dangling reference" }
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
new file mode 100644
index 00000000000..4b0de2d8670
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+int& ref(const int&);
+int&& rref(const int&);
+
+void
+g ()
+{
+ const int& r1 = ref (1); // { dg-warning "dangling reference" }
+ int& r2 = ref (2); // { dg-bogus "dangling reference" }
+ auto& r3 = ref (3); // { dg-bogus "dangling reference" }
+ int&& r4 = rref (4); // { dg-warning "dangling reference" }
+ auto&& r5 = rref (5); // { dg-warning "dangling reference" }
+ const int&& r6 = rref (6); // { dg-warning "dangling reference" }
+}
base-commit: 0a7b437ca71e2721e9bcf070762fc54ef7991aeb
--
2.38.1
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] c++: Disable -Wdangling-reference when initing T&
2022-11-11 20:22 [PATCH] c++: Disable -Wdangling-reference when initing T& Marek Polacek
@ 2022-11-14 23:16 ` Jason Merrill
0 siblings, 0 replies; 2+ messages in thread
From: Jason Merrill @ 2022-11-14 23:16 UTC (permalink / raw)
To: Marek Polacek, GCC Patches
On 11/11/22 10:22, Marek Polacek wrote:
> Non-const lvalue references can't bind to a temporary, so the
> warning should not be emitted if we're initializing something of that
> type. I'm not disabling the warning when the function itself returns
> a non-const lvalue reference, that would regress at least
>
> const int &r = std::any_cast<int&>(std::any());
>
> in Wdangling-reference2.C where the any_cast returns an int&.
>
> Unfortunately, this patch means we'll stop diagnosing
>
> int& fn(int&& x) { return static_cast<int&>(x); }
> void test ()
> {
> int &r = fn(4);
> }
>
> where there's a genuine dangling reference. OTOH, the patch
> should suppress false positives with iterators, like:
>
> auto &candidate = *candidates.begin ();
>
> and arguably that's more important than detecting some relatively
> obscure cases. It's probably not worth it making the warning more
> complicated by, for instance, not warning when a fn returns 'int&'
> but takes 'const int&' (because then it can't return its argument).
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK.
> gcc/cp/ChangeLog:
>
> * call.cc (maybe_warn_dangling_reference): Don't warn when initializing
> a non-const lvalue reference.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp23/elision4.C: Remove dg-warning.
> * g++.dg/warn/Wdangling-reference1.C: Turn dg-warning into dg-bogus.
> * g++.dg/warn/Wdangling-reference7.C: New test.
> ---
> gcc/cp/call.cc | 10 ++++++++--
> gcc/testsuite/g++.dg/cpp23/elision4.C | 4 ++--
> gcc/testsuite/g++.dg/warn/Wdangling-reference1.C | 4 ++--
> gcc/testsuite/g++.dg/warn/Wdangling-reference7.C | 16 ++++++++++++++++
> 4 files changed, 28 insertions(+), 6 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index bd3b64a7e26..ef618d5c485 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13679,8 +13679,14 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
> {
> if (!warn_dangling_reference)
> return;
> - if (!(TYPE_REF_OBJ_P (TREE_TYPE (decl))
> - || std_pair_ref_ref_p (TREE_TYPE (decl))))
> + tree type = TREE_TYPE (decl);
> + /* Only warn if what we're initializing has type T&& or const T&, or
> + std::pair<const T&, const T&>. (A non-const lvalue reference can't
> + bind to a temporary.) */
> + if (!((TYPE_REF_OBJ_P (type)
> + && (TYPE_REF_IS_RVALUE (type)
> + || CP_TYPE_CONST_P (TREE_TYPE (type))))
> + || std_pair_ref_ref_p (type)))
> return;
> /* Don't suppress the diagnostic just because the call comes from
> a system header. If the DECL is not in a system header, or if
> diff --git a/gcc/testsuite/g++.dg/cpp23/elision4.C b/gcc/testsuite/g++.dg/cpp23/elision4.C
> index d39053ad741..77dcffcdaad 100644
> --- a/gcc/testsuite/g++.dg/cpp23/elision4.C
> +++ b/gcc/testsuite/g++.dg/cpp23/elision4.C
> @@ -34,6 +34,6 @@ T& temporary2(T&& x) { return static_cast<T&>(x); }
> void
> test ()
> {
> - int& r1 = temporary1 (42); // { dg-warning "dangling reference" }
> - int& r2 = temporary2 (42); // { dg-warning "dangling reference" }
> + int& r1 = temporary1 (42);
> + int& r2 = temporary2 (42);
> }
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> index 97c81ee716c..1718c28165e 100644
> --- a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> @@ -139,6 +139,6 @@ struct Y {
> // x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
> int&& x1 = Y(); // { dg-warning "dangling reference" }
> int&& x2 = Y{}; // { dg-warning "dangling reference" }
> -int& x3 = Y(); // { dg-warning "dangling reference" }
> -int& x4 = Y{}; // { dg-warning "dangling reference" }
> +int& x3 = Y(); // { dg-bogus "dangling reference" }
> +int& x4 = Y{}; // { dg-bogus "dangling reference" }
> const int& t1 = Y().foo(10); // { dg-warning "dangling reference" }
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
> new file mode 100644
> index 00000000000..4b0de2d8670
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C
> @@ -0,0 +1,16 @@
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +int& ref(const int&);
> +int&& rref(const int&);
> +
> +void
> +g ()
> +{
> + const int& r1 = ref (1); // { dg-warning "dangling reference" }
> + int& r2 = ref (2); // { dg-bogus "dangling reference" }
> + auto& r3 = ref (3); // { dg-bogus "dangling reference" }
> + int&& r4 = rref (4); // { dg-warning "dangling reference" }
> + auto&& r5 = rref (5); // { dg-warning "dangling reference" }
> + const int&& r6 = rref (6); // { dg-warning "dangling reference" }
> +}
>
> base-commit: 0a7b437ca71e2721e9bcf070762fc54ef7991aeb
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