On 11/9/22 15:56, Marek Polacek wrote:
This patch extends -Wdangling-reference to also warn for
auto v = std::minmax(1, 2);
which dangles because this overload of std::minmax returns
a std::pair<const int&, const int&> where the two references are
bound to the temporaries created for the arguments of std::minmax.
This is a common footgun, also described at
<https://en.cppreference.com/w/cpp/algorithm/minmax> in Notes.
It works by extending do_warn_dangling_reference to also warn when the
function returns a std::pair<const T&, const T&>. std_pair_ref_ref_p
is a new helper to check that.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
gcc/cp/ChangeLog:
* call.cc (std_pair_ref_ref_p): New.
(do_warn_dangling_reference): Also warn when the function returns
std::pair<const T&, const T&>. Recurse into TARGET_EXPR_INITIAL.
(maybe_warn_dangling_reference): Don't return early if we're
initializing a std_pair_ref_ref_p.
gcc/ChangeLog:
* doc/gcc/gcc-command-options/options-controlling-c++-dialect.rst:
Extend the description of -Wdangling-reference.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference6.C: New test.
---
gcc/cp/call.cc | 52 ++++++++++++++++---
.../options-controlling-c++-dialect.rst | 10 ++++
.../g++.dg/warn/Wdangling-reference6.C | 38 ++++++++++++++
3 files changed, 94 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference6.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 492db9b59ad..bd3b64a7e26 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13527,6 +13527,34 @@ initialize_reference (tree type, tree expr,
return expr;
}
+/* Return true if T is std::pair<const T&, const T&>. */
+
+static bool
+std_pair_ref_ref_p (tree t)
+{
+ /* First, check if we have std::pair. */
+ if (!NON_UNION_CLASS_TYPE_P (t)
+ || !CLASSTYPE_TEMPLATE_INSTANTIATION (t))
+ return false;
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (t));
+ if (!decl_in_std_namespace_p (tdecl))
+ return false;
+ tree name = DECL_NAME (tdecl);
+ if (!name || !id_equal (name, "pair"))
+ return false;
+
+ /* Now see if the template arguments are both const T&. */
+ tree args = CLASSTYPE_TI_ARGS (t);
+ if (TREE_VEC_LENGTH (args) != 2)
+ return false;
+ for (int i = 0; i < 2; i++)
+ if (!TYPE_REF_OBJ_P (TREE_VEC_ELT (args, i))
+ || !CP_TYPE_CONST_P (TREE_TYPE (TREE_VEC_ELT (args, i))))
+ return false;
+
+ return true;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13556,11 +13584,6 @@ do_warn_dangling_reference (tree expr)
|| warning_suppressed_p (fndecl, OPT_Wdangling_reference)
|| !warning_enabled_at (DECL_SOURCE_LOCATION (fndecl),
OPT_Wdangling_reference)
- /* If the function doesn't return a reference, don't warn. This
- can be e.g.
- const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
- which doesn't dangle: std::min here returns an int. */
- || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl)))
/* Don't emit a false positive for:
std::vector<int> v = ...;
std::vector<int>::const_iterator it = v.begin();
@@ -13573,6 +13596,20 @@ do_warn_dangling_reference (tree expr)
&& DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
return NULL_TREE;
+ tree rettype = TREE_TYPE (TREE_TYPE (fndecl));
+ /* If the function doesn't return a reference, don't warn. This
+ can be e.g.
+ const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
+ which doesn't dangle: std::min here returns an int.
+
+ If the function returns a std::pair<const T&, const T&>, we
+ warn, to detect e.g.
+ std::pair<const int&, const int&> v = std::minmax(1, 2);
+ which also creates a dangling reference, because std::minmax
+ returns std::pair<const T&, const T&>(b, a). */
+ if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
The patch is OK, but do you want to check reference to const for the
single ref case as well, while you're changing this?
+ return NULL_TREE;
+
/* Here we're looking to see if any of the arguments is a temporary
initializing a reference parameter. */
for (int i = 0; i < call_expr_nargs (expr); ++i)
@@ -13614,6 +13651,8 @@ do_warn_dangling_reference (tree expr)
return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
case PAREN_EXPR:
return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ case TARGET_EXPR:
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
default:
return NULL_TREE;
}
@@ -13640,7 +13679,8 @@ maybe_warn_dangling_reference (const_tree decl, tree
init)
{
if (!warn_dangling_reference)
return;
- if (!TYPE_REF_P (TREE_TYPE (decl)))
+ if (!(TYPE_REF_OBJ_P (TREE_TYPE (decl))
And here.
+ || std_pair_ref_ref_p (TREE_TYPE (decl))))
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/doc/gcc/gcc-command-options/options-controlling-c++-dialect.rst
b/gcc/doc/gcc/gcc-command-options/options-controlling-c++-dialect.rst
index 5b05d31aae9..8d2a2789ef6 100644
--- a/gcc/doc/gcc/gcc-command-options/options-controlling-c++-dialect.rst
+++ b/gcc/doc/gcc/gcc-command-options/options-controlling-c++-dialect.rst
@@ -855,6 +855,16 @@ In addition, these warning options have meanings only for
C++ programs:
const T& foo (const T&) { ... }
#pragma GCC diagnostic pop
+ :option:`-Wdangling-reference` also warns about code like
+
+ .. code-block:: c++
+
+ auto p = std::minmax(1, 2);
+
+ where ``std::minmax`` returns ``std::pair<const int&, const int&>``, and
+ both references dangle after the end of the full expression that contains
+ the call to `std::minmax``.
+
This warning is enabled by :option:`-Wall`.
.. option:: -Wno-dangling-reference
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference6.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference6.C
new file mode 100644
index 00000000000..bf849e290d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference6.C
@@ -0,0 +1,38 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wdangling-reference" }
+// Test -Wdangling-reference with std::minmax.
+
+#include <algorithm>
+
+using U = std::pair<const int&, const int&>;
+
+int
+fn1 ()
+{
+ std::pair<const int&, const int&> v = std::minmax(1, 2); // { dg-warning
"dangling reference" }
+ U v2 = std::minmax(1, 2); // { dg-warning "dangling reference" }
+ auto v3 = std::minmax(1, 2); // { dg-warning "dangling reference" }
+ return v.first + v2.second + v3.first;
+}
+
+int
+fn2 ()
+{
+ int n = 1;
+ auto p = std::minmax(n, n + 1); // { dg-warning "dangling reference" }
+ int m = p.first; // ok
+ int x = p.second; // undefined behavior
+
+ // Note that structured bindings have the same issue
+ auto [mm, xx] = std::minmax(n, n + 1); // { dg-warning "dangling reference" }
+ (void) xx; // undefined behavior
+
+ return m + x;
+}
+
+int
+fn3 ()
+{
+ auto v = std::minmax({1, 2});
+ return v.first;
+}
base-commit: 1cdfd0e5cd5fc1f493d0832ed65d31320f9585b7
