P3596R0 — Undefined Behavior and IFNDR Annexes

P3596R0 — Undefined Behavior and IFNDR Annexes (18 items) CWG

Joshua Berne, Timur Doumler, Jens Maurer, Shafik Yaghmour

This paper provides wording to add two new informative annexes to the C++ Standard: one cataloging all instances of core undefined behavior (Annex E) and one cataloging all instances of ill-formed, no diagnostic required (IFNDR) behavior (Annex F). Each entry includes a title, a cross-reference to the normative text, a summary, and a code example. The annexes also introduce stable identifiers for each kind of undefined or IFNDR behavior to serve as common vocabulary for future mitigation efforts.

Section (D+a).2.1 [ub.intro.object], paragraph 4, page 54 — Function declaration *make_y() is missing its return type; should be Y* make_y() since the function returns a Y*. [4] [1]
Section (D+b).6.6.3 [ifndr.dcl.attr.noreturn], paragraph 2, page 91 — TU#1 declares void f() (zero parameters) while TU#2 declares void f(int i) (one parameter); these are different functions, so the example does not demonstrate the IFNDR condition (same function declared with and without [[noreturn]] across TUs). TU#2 should declare void f(). [2]
Section (D+a).4.2 [ub.return.coroutine], paragraph 2, page 74 — Example mixes std::coroutine_handle (in resumable) with std::experimental::coroutine_handle and std::experimental::suspend_always (in promise_type); these are from different headers and namespaces and the code does not compile as written. [8] [3]
Section (D+a).3.2 [ub.basic.lval], paragraph 1, page 63 — Cross-reference [conv.rval] does not exist as a stable name; the correct reference for the similar relation is [conv.qual], consistent with the paper's own normative wording on page 22. [5] [4]
Section (D+a).3.18 [ub.expr.add], paragraph 5, page 72 — Same erroneous cross-reference [conv.rval]; should be [conv.qual], consistent with the paper's own normative text on page 31. [5]
Section 7.6.1.9 [expr.static.cast], paragraph 11, page 27 — Stable name contains orignal (misspelling of original): [expr.static.cast.does.not.contain.orignal.member]. [7] [6]
Section (D+b).3.1 [ifndr.basic.link], paragraph 2, page 85 — Code example uses moodule (not a C++ keyword); should be module. As written the example does not declare a module interface unit. [7]
Sections (D+b).6.1.1, (D+b).6.1.2, (D+b).6.2.1, (D+b).6.3.1, (D+b).6.4.1, (D+b).6.5, (D+b).6.6.2, (D+b).7.2, (D+b).10.8.1, (D+b).10.8.2.2, (D+b).10.8.4, pages 89-99 — At least 12 IFNDR annex sections contain empty examples ([Example: immediately followed by end example]) with unresolved TODO notes. [8]
Section (D+a).2.12 [ub.basic.start.term], paragraph 4, page 62 — UB annex entry for signal handler UB contains an empty example with no code content. [9]
Section 1, page 2 — Subject-verb disagreement: Calls ... was made should be Calls ... were made. [10]
Section (D+a).2.7, page 59 — implemention-defined is misspelled; should be implementation-defined. [11]
Section (D+b).6.6.3 [ifndr.dcl.attr.noreturn], paragraph 1, page 91 — requried is misspelled; should be required. [12]
Section (D+a).3.6 [ub.conv.fpint], paragraph 4, page 65 — represetable is misspelled; should be representable. [6] [13]
Section (D+b).11.2 [ifndr.cpp.include], paragraph 2, page 100 — diagnoatic in code comment is misspelled; should be diagnostic. [14]
Section 1, page 2 — atempted is misspelled; should be attempted. [1] [15]
Section 2, page 3 — lke in editorial note is misspelled; should be like. [2] [16]
Section 2, page 3 — margines is misspelled; should be margins. [3] [17]

References — Anthropic Citations API

[1]
"We have atempted to follow some basic guidance on what is and is not appropriate to include in this annex: • In general the annex examples must be correct and provide an example of the specific..."

[2]
"JMB: Such comments will appear in the text in a box lke this one, and must be resolved prior to the completion of wording review. "

[3]
"In the final rendered standard these will be annexes E and F. Annexes that come after will then have their values increased, and so the current Annex E (Conformance with UAX #31) will become..."

[4]
"Calls to formalize this data into an annex within the C++ Standard itself was made in [P2234R0]"

[5]
"Most other uses of such a pointer have implemention-defined behavior."

[6]
"X* p = (X*)std::malloc(sizeof(struct X)); p->a = 1; // undefined behavior, no set of objects give us defined behavior return p; } — end example] [Example: #include struct X { int..."

[7]
"module; export moodule M; export int h(); // #1, attached to M"

[8]
"1 Specified in: [expr.basic.lvalue.strict.aliasing.violation] If a program attempts to access([defns.access]) the stored value of an object whose dynamic type is T through a glvalue whose type..."

[9]
"where T and the array element type are not similar([conv.rval]), the behavior is undefined."

[10]
"No diagnostic is requried if a function is declared in one translation unit with the noreturn attribute"

[11]
"// Ill-formed no diagnoatic required"

[12]
"4 [Example: int main() { __uint128_t x2 = -1; float f = x2; // undefined behavior on systems where the range of // representable values of float is [-max,+max] on system where // represetable..."

[13]
"Otherwise, the behavior is undefined([expr.static.cast.does.not.contain.orignal.member]). "

[14]
"Translation unit #1 [[noreturn]] void f() {} Translation unit #2 void f(int i); // ill-formed no diagnostic required, declared without noreturn"

[15]
"2 [Example: #include #include #include #include class resumable { public: struct promise_type; using coro_handle =..."

[16]
"[Example: — end example] JMB: produce an example"

[17]
"[Example: — end example] JMB/TD: This is really a general precondition imposed on the Standard Library, not a piece of core language undefined behavior. It is also currently missing an example."

Summary: P3596R0 proposes two new informative annexes for the C++ standard: one cataloguing every instance of undefined behavior with cross-references and examples, and one doing the same for ill-formed, no diagnostic required (IFNDR) cases. The paper includes proposed normative wording changes to tag each UB and IFNDR site with a stable name, plus draft annex content with explanatory examples.

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