[Issue 24834] New: opAssign is not generated for structs with copy constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24834

          Issue ID: 24834
           Summary: opAssign is not generated for structs with copy
                    constructors
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: issues.dlang at jmdavisProg.com

If a struct has a postblit constructor but no opAssign, then the compiler will
generate an opAssign which calls the postblit constructor so that the
assignment behavior matches the copying behavior, e.g.

---
struct S
{
    this(this) {}
}
pragma(msg, __traits(allMembers, S));
---

prints

---
AliasSeq!("__postblit", "__xpostblit", "opAssign")
---

However, copy constructors do not get such an opAssign, e.g.

---
struct S
{
    this(ref S) {}
}
pragma(msg, __traits(allMembers, S));
---

prints

---
AliasSeq!("__ctor")
---

This means that in order for the assignment behavior to match the copying
behavior, the user must explicitly declare an opAssign.

https://dlang.org/spec/struct.html#assign-overload explicitly mentions that
structs with postblit constructors will have an opAssign generated if one is
not declared, but it does not mention anything about copy constructors. So,
technically, the current behavior does not violate the spec. However, it's an
inconsistency with postblit constructors, and I see no reason why copy
constructors should not be treated the same as postblit constructors in this
respect. The logic for why postblit constructors get an opAssign seems like it
would apply equally to copy constructors.

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