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[spec/legacy] Assigning to struct rvalue #4349

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[spec/legacy] Assigning to struct rvalue #4349

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31 changes: 31 additions & 0 deletions spec/legacy.dd
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,8 @@ $(COMMENT
$(TROW Escaping $(DDSUBLINK spec/attribute, scope, `scope`) data,
`scope` is $(LINK2 https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1000.md,
enforced) in `@safe` code, 2024)
$(TROW $(RELATIVE_LINK2 struct-assign, Assigning to struct rvalue),
Disallow for structs which overload e.g. `opAssign`, 2024)

$(COMMENT Not enforced yet)
$(TROW `alias` target first syntax, use `alias name = target` instead.)
Expand Down Expand Up @@ -93,6 +95,35 @@ struct Foo
}
---

$(H2 $(LNAME2 struct-assign, Assigning to struct rvalue))

$(P It has always been an error for POD structs to assign from an rvalue.
From the 2024 edition, it is also an error to assign from a struct rvalue when
it would call `opAssign`, `opOpAssign`, `opUnary!"++"`, or `opUnary!"--"`.)

---
struct S
{
int i;
void opAssign(S s);
}

S foo() => S.init;

void main()
{
foo() = S(2); // no-op
}
---
$(P Unless an overloaded assignment has side-effects, the assignment
will have no effect and indicates a bug.)

$(H3 Corrective Action)

$(P If side-effects are needed, call e.g.
`opAssign` directly. Note: Calling a non-const method on a struct rvalue
$(DDSUBLINK spec/struct, member-functions, is allowed).)


$(SPEC_SUBNAV_PREV_NEXT glossary, Glossary, editions, Editions)
)
Expand Down