[Momen-G-Ar]

Summary

Fixes GH #324 (discussion): when the default member initializer of an aggregate class aggregate-initializes another object of the same class, the implicit this in the elements of that nested initializer referred to the nested object instead of to the object being initialized.

struct Y { unsigned k; unsigned n = Y{2, k}.n; };  // n = this->k
int main() {
  Y *p = new Y{1};
  if (p->n != 1) abort();  // EDG: p->n == 2
}

k here is this->k, and this is the aggregate whose element is initialized from the default member initializer ([dcl.init.aggr]/5.1). g++ and Clang give n == 1.

Initialization	Before	After
Y y{1};, Y a = Y{1}; (folded to a constant by the front end)	correct	unchanged
new Y{1}, return Y{1};, inner object passed by value or by const Y & (lowered)	this bound to the nested Y{2, k}	correct

Same result in strict, --g++ and --clang modes.

Root cause

lower_param_ref (lower_il.c) replaces the enk_param_ref for this in a non-constant aggregate initialization with the innermost entry on aggregate_this_stack whose type matches. A comment there assumed that "there can only be one". In the example, the inner Y{2, k} pushes a second Y entry (in lower_dynamic_init_aggregate_constant, lower_init.c) on top of the outer one before its explicit element k is lowered, so this bound to the inner temporary.

Change

Only an element that comes from a default member initializer, which the front end marks with implicit_aggr_element, can refer to the aggregate it initializes. An element written in the braces refers to the this of the enclosing context. The constexpr interpreter already uses this rule (interpret.c, set_up_param_ref_for_this_ptr is called only for implicit_aggr_element elements).

• lower_il.h: new an_init_pos_descr::not_this_candidate flag, cleared in clear_init_pos_descr.
• lower_init.c: lower_dynamic_init_aggregate_constant sets the flag on its stack entry while an explicit element is lowered. Anonymous unions are not affected, since their members' default member initializers refer to the enclosing class.
• lower_il.c: the stack search moves to replace_param_ref_with_aggregate_this. It skips flagged entries first; if nothing matches, it repeats the previous search, so any case not covered gives the same result as before.

Changes entry: src/pending_changes/gh-324-nsdmi-this-nested-aggregate.txt (passes edg-collect-changes --check). Based on main at 35b7c47.

Tests

Test	Type	Checks
regressions/gh-324.sft.cpp	rp	new, return, by-value and by-reference cases; folded Y y{1}; the inner object's own default member initializer still binds to the inner object (W); anonymous union, aggregate base class, lambda capturing this, array new. --c++20 strict, --g++ 15, --clang

The W check is skipped in --clang mode: there, a member skipped by a designated initializer does not get its default member initializer (W{.k = 2, .n = 0}.j is 0). That happens without this change too and is the issue in discussion #259.

• The new test fails on main and passes with this change (edg-docker-test, edg_x86_64 and edg_x86_64_cp, all three variants).
• The recordings were created with edg-docker-test -W, and the default.* files were reviewed.
• 320 related tests (NSDMI, aggregate, designated initializer, lambda this) match their expectations in edg_x86_64 and edg_x86_64_cp.
• Full suite on GitHub Actions (Build & Test CI, all four configurations): run 40556389927.

Discussion: edgcpp#324

[Momen-G-Ar]

Merged upstream as edgcpp#408 (e6d9621).