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[spec/legacy] Assigning to struct rvalue #4349

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dlang-bot merged 2 commits into from
Jan 9, 2026
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[spec/legacy] Assigning to struct rvalue #4349

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09d5630
[spec/legacy] Assigning to struct rvalue
ntrel Dec 13, 2025
8e9d1bd
Restrict error to discarded assignment to struct without pointers
ntrel Jan 7, 2026
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37 changes: 37 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 +95,41 @@ struct Foo
}
---

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

$(P It has always been an error for $(DDSUBLINK spec/struct, POD, POD) structs
to assign from an rvalue.)

$(P From the 2024 edition, it is an error to discard the result of an
assignment from a struct rvalue when it would call `opAssign`, `opOpAssign`,
`opUnary!"++"`, or `opUnary!"--"` and the struct has no pointer
fields:)

---
struct S
{
int i;

void opAssign(S s);
}

S foo();

void main()
{
foo() = S(2); // Error, possible no-op
}
---
$(P Above, unless `opAssign` mutates global data, the assignment in `main`
will have no effect and indicates a bug.)

$(H3 Corrective Action)

$(P If a struct rvalue assignment is needed to mutate global state, either
call the operator overload method directly or use an lvalue.
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)
)
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