This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-12-20
[Moved to DR at the February, 2014 meeting.]
Consider an example like the following:
struct Base { virtual int call() = 0; }; Base *foo() { constexpr int x = 0; struct Local : Base { virtual int call() { return x; } }; static Local local; return &local; } int main() { return foo()->call(); }While the likely intention is that the lvalue-to-rvalue conversion ofthe block-scope constant is implemented by using the value of the constantexpression in place of reading from storage, it seems that the wording of7.3.2 [conv.lval] paragraph 2 does not prevent this program frombeing subject to undefined behaviour caused by lifetime violation. Inparticular, it seems that a name expression that appears in apotentially-evaluated expression such that the object named is not odr-used(by that instance of the name) may still be evaluated, in theory, as anlvalue through which the object named or a subobject thereof isaccessed.
Proposed resolution (September, 2013):
Change 7.3.2 [conv.lval] paragraph 2 as follows:
When an lvalue-to-rvalue conversion
occurs in an unevaluatedoperand or a subexpression thereof (Clause 7 [expr])is applied to an expressione, and either
e is not potentially evaluated, or
the evaluation ofe results in the evaluation of amemberex of the set of potential results ofe, andex names a variablex that is not odr-used byex (6.3 [basic.def.odr]),
the value contained in the referenced object is not accessed.[Example:
struct S { int n; }; auto f() { S x { 1 }; constexpr S y { 2 }; return [&](bool b) { return (b ? y : x).n; }; } auto g = f(); int m = g(false); // undefined behavior due to access ofx.n outside its lifetime int n = g(true); // OK, does not accessy.n—end example] In all other cases, the resultof the conversion...