diff options
Diffstat (limited to 'gcc/cp/call.cc')
| -rw-r--r-- | gcc/cp/call.cc | 148 |
1 files changed, 136 insertions, 12 deletions
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index 6a34e9c..951b9fd 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -9313,6 +9313,16 @@ conv_binds_ref_to_prvalue (conversion *c) return conv_is_prvalue (next_conversion (c)); } +/* True iff EXPR represents a (subobject of a) temporary. */ + +static bool +expr_represents_temporary_p (tree expr) +{ + while (handled_component_p (expr)) + expr = TREE_OPERAND (expr, 0); + return TREE_CODE (expr) == TARGET_EXPR; +} + /* True iff C is a conversion that binds a reference to a temporary. This is a superset of conv_binds_ref_to_prvalue: here we're also interested in xvalues. */ @@ -9330,18 +9340,14 @@ conv_binds_ref_to_temporary (conversion *c) struct Derived : Base {}; const Base& b(Derived{}); where we bind 'b' to the Base subobject of a temporary object of type - Derived. The subobject is an xvalue; the whole object is a prvalue. */ - if (c->kind != ck_base) - return false; - c = next_conversion (c); - if (c->kind == ck_identity && c->u.expr) - { - tree expr = c->u.expr; - while (handled_component_p (expr)) - expr = TREE_OPERAND (expr, 0); - if (TREE_CODE (expr) == TARGET_EXPR) - return true; - } + Derived. The subobject is an xvalue; the whole object is a prvalue. + + The ck_base doesn't have to be present for cases like X{}.m. */ + if (c->kind == ck_base) + c = next_conversion (c); + if (c->kind == ck_identity && c->u.expr + && expr_represents_temporary_p (c->u.expr)) + return true; return false; } @@ -13428,6 +13434,121 @@ initialize_reference (tree type, tree expr, return expr; } +/* 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 + if none found. For instance: + + const S& s = S().self(); // S::self (&TARGET_EXPR <...>) + const int& r = (42, f(1)); // f(1) + const int& t = b ? f(1) : f(2); // f(1) + const int& u = b ? f(1) : f(g); // f(1) + const int& v = b ? f(g) : f(2); // f(2) + const int& w = b ? f(g) : f(g); // NULL_TREE + const int& y = (f(1), 42); // NULL_TREE + const int& z = f(f(1)); // f(f(1)) + + EXPR is the initializer. */ + +static tree +do_warn_dangling_reference (tree expr) +{ + STRIP_NOPS (expr); + switch (TREE_CODE (expr)) + { + case CALL_EXPR: + { + tree fndecl = cp_get_callee_fndecl_nofold (expr); + if (!fndecl + || 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)))) + 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) + { + tree arg = CALL_EXPR_ARG (expr, i); + /* Check that this argument initializes a reference, except for + the argument initializing the object of a member function. */ + if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) + && !TYPE_REF_P (TREE_TYPE (arg))) + continue; + /* It could also be another call taking a temporary and returning + it and initializing this reference parameter. */ + if (do_warn_dangling_reference (arg)) + return expr; + STRIP_NOPS (arg); + if (TREE_CODE (arg) == ADDR_EXPR) + arg = TREE_OPERAND (arg, 0); + if (expr_represents_temporary_p (arg)) + return expr; + /* Don't warn about member function like: + std::any a(...); + S& s = a.emplace<S>({0}, 0); + which constructs a new object and returns a reference to it, but + we still want to detect: + struct S { const S& self () { return *this; } }; + const S& s = S().self(); + where 's' dangles. If we've gotten here, the object this function + is invoked on is not a temporary. */ + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)) + break; + } + return NULL_TREE; + } + case COMPOUND_EXPR: + return do_warn_dangling_reference (TREE_OPERAND (expr, 1)); + case COND_EXPR: + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1))) + return t; + return do_warn_dangling_reference (TREE_OPERAND (expr, 2)); + case PAREN_EXPR: + return do_warn_dangling_reference (TREE_OPERAND (expr, 0)); + default: + return NULL_TREE; + } +} + +/* Implement -Wdangling-reference, to detect cases like + + int n = 1; + const int& r = std::max(n - 1, n + 1); // r is dangling + + This creates temporaries from the arguments, returns a reference to + one of the temporaries, but both temporaries are destroyed at the end + of the full expression. + + This works by checking if a reference is initialized with a function + that returns a reference, and at least one parameter of the function + is a reference that is bound to a temporary. It assumes that such a + function actually returns one of its arguments. + + DECL is the reference being initialized, INIT is the initializer. */ + +static void +maybe_warn_dangling_reference (const_tree decl, tree init) +{ + if (!warn_dangling_reference) + return; + if (!TYPE_REF_P (TREE_TYPE (decl))) + return; + if (tree call = do_warn_dangling_reference (init)) + { + auto_diagnostic_group d; + if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference, + "possibly dangling reference to a temporary")) + inform (EXPR_LOCATION (call), "the temporary was destroyed at " + "the end of the full expression %qE", call); + } +} + /* If *P is an xvalue expression, prevent temporary lifetime extension if it gets used to initialize a reference. */ @@ -13525,6 +13646,9 @@ extend_ref_init_temps (tree decl, tree init, vec<tree, va_gc> **cleanups, tree type = TREE_TYPE (init); if (processing_template_decl) return init; + + maybe_warn_dangling_reference (decl, init); + if (TYPE_REF_P (type)) init = extend_ref_init_temps_1 (decl, init, cleanups, cond_guard); else |