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))) + 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)) + || 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 -- 2.38.1
