On 1/18/23 12:52, Marek Polacek wrote:
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
So I figured that perhaps we want to look at the object we're invoking
the member function(s) on and see if that is a temporary, as in, don't
warn about
const Plane & meta = fm.planes().inner();
but do warn about
const Plane & meta = FrameMetadata().planes().inner();
It's ugly, but better than asking users to add #pragmas into their code.
Hmm, that doesn't seem right; the former is only OK because Ref is in
fact a reference-like type. If planes() returned a class that held
data, we would want to warn.
In this case, we might recognize the reference-like class because it has
a reference member and a constructor taking the same reference type.
That wouldn't help with std::reference_wrapper or std::ref_view because
they have pointer members instead of references, but perhaps loosening
the check to include that case would make sense?
Jason
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (do_warn_dangling_reference): Don't warn when the
object member functions are invoked on is not a temporary.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
---
gcc/cp/call.cc | 33 +++++++-
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
2 files changed, 109 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 0780b5840a3..43e65c3dffb 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13850,7 +13850,38 @@ do_warn_dangling_reference (tree expr)
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
if (expr_represents_temporary_p (arg))
- return expr;
+ {
+ /* An ugly attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Don't warn about s.a().b() but do warn about S().a().b(),
+ supposing that the member function is returning a reference
+ to a subobject of the (non-temporary) object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && i == 0)
+ {
+ tree t = arg;
+ while (handled_component_p (t))
+ t = TREE_OPERAND (t, 0);
+ t = TARGET_EXPR_INITIAL (arg);
+ /* Quite likely we don't have a chain of member functions
+ (like a().b().c()). */
+ if (TREE_CODE (t) != CALL_EXPR)
+ return expr;
+ /* Walk the call chain to the original object and see if
+ it was a temporary. */
+ do
+ t = tree_strip_nop_conversions (CALL_EXPR_ARG (t, 0));
+ while (TREE_CODE (t) == CALL_EXPR);
+ /* If the object argument is &TARGET_EXPR<>, we've started
+ off the chain with a temporary and we want to warn. */
+ if (TREE_CODE (t) == ADDR_EXPR)
+ t = TREE_OPERAND (t, 0);
+ if (!expr_represents_temporary_p (t))
+ break;
+ }
+ return expr;
+ }
/* Don't warn about member function like:
std::any a(...);
S& s = a.emplace<S>({0}, 0);
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..32280f3e282
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning
"dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling
reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling
reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference"
}
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner(); // { dg-warning "dangling reference"
}
+ (void) d8;
+}
base-commit: c6a011119bfa038ccbfc9f123ede14a3d6237fab
