From 9010fdba68063beccfbab0aa9ec8739f232ca2f4 Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Fri, 19 Jan 2024 13:59:41 -0500
Subject: [PATCH] c++: -Wdangling-reference and lambda false warning [PR109640]
-Wdangling-reference checks if a function receives a temporary as its
argument, and only warns if any of the arguments was a temporary. But
we should not warn when the temporary represents a lambda or we generate
false positives as in the attached testcases.
PR c++/113256
PR c++/111607
PR c++/109640
gcc/cp/ChangeLog:
* call.cc (do_warn_dangling_reference): Don't warn if the temporary
is of lambda type.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference14.C: New test.
* g++.dg/warn/Wdangling-reference15.C: New test.
* g++.dg/warn/Wdangling-reference16.C: New test.
---
gcc/cp/call.cc | 9 ++++--
.../g++.dg/warn/Wdangling-reference14.C | 22 +++++++++++++
.../g++.dg/warn/Wdangling-reference15.C | 31 +++++++++++++++++++
.../g++.dg/warn/Wdangling-reference16.C | 13 ++++++++
4 files changed, 72 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference14.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference15.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference16.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 1f5ff417c811..77f51bacce3c 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14123,7 +14123,10 @@ do_warn_dangling_reference (tree expr, bool arg_p)
tree e = expr;
while (handled_component_p (e))
e = TREE_OPERAND (e, 0);
- if (!reference_like_class_p (TREE_TYPE (e)))
+ tree type = TREE_TYPE (e);
+ /* If the temporary represents a lambda, we don't really know
+ what's going on here. */
+ if (!reference_like_class_p (type) && !LAMBDA_TYPE_P (type))
return expr;
}
@@ -14180,10 +14183,10 @@ do_warn_dangling_reference (tree expr, bool arg_p)
initializing this reference parameter. */
if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
- /* Don't warn about member function like:
+ /* Don't warn about member functions like:
std::any a(...);
S& s = a.emplace<S>({0}, 0);
- which constructs a new object and returns a reference to it, but
+ which construct a new object and return a reference to it, but
we still want to detect:
struct S { const S& self () { return *this; } };
const S& s = S().self();
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C
new file mode 100644
index 000000000000..92b38a965e0a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C
@@ -0,0 +1,22 @@
+// PR c++/113256
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <utility>
+#include <cassert>
+
+template<class M, class T, class A> auto bind(M T::* pm, A)
+{
+ return [=]( auto&& x ) -> M const& { return x.*pm; };
+}
+
+template<int I> struct arg {};
+
+arg<1> _1;
+
+int main()
+{
+ std::pair<int, int> pair;
+ int const& x = bind( &std::pair<int, int>::first, _1 )( pair ); // { dg-bogus "dangling reference" }
+ assert( &x == &pair.first );
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C
new file mode 100644
index 000000000000..c39577db64a6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C
@@ -0,0 +1,31 @@
+// PR c++/111607
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <variant>
+
+struct S {
+ constexpr S(int i_) : i(i_) {}
+ S(S const &) = delete;
+ S & operator=(S const &) = delete;
+ S(S &&) = delete;
+ S & operator=(S &&) = delete;
+ int i;
+};
+
+struct A {
+ S s{0};
+};
+
+using V = std::variant<A>;
+
+consteval auto f(V const & v) {
+ auto const & s = std::visit([](auto const & v) -> S const & { return v.s; }, v); // { dg-bogus "dangling reference" }
+ return s.i;
+}
+
+int main() {
+ constexpr V a{std::in_place_type<A>};
+ constexpr auto i = f(a);
+ return i;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C
new file mode 100644
index 000000000000..91996922291b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C
@@ -0,0 +1,13 @@
+// PR c++/109640
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+bool
+fn0 ()
+{
+ int a;
+ int&& i = [](int& r) -> int&& { return static_cast<int&&>(r); }(a); // { dg-bogus "dangling reference" }
+ auto const l = [](int& r) -> int&& { return static_cast<int&&>(r); };
+ int&& j = l(a);
+ return &i == &j;
+}
--
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