Bug 9597 – using "this" as a type leads to confusion

Using "this" as a type in method parameter declarations is not universal because of the postblit constructor signature. It also creates confusion when the method signature resembles the postblit constructor. Here are a few examples of what can and cannot be declared (DMD 2.062): ----- struct S { int x = 1; int y = 0; this (this) // postblit constructor called like this: "S b = a;" { x = 10; y = 1; } this (ref this) // constructor called as: "S c = S (a);" { x = 100; } this (S q) // compiles, example call: "S d = S (S (a));" { x = 1000; } this (ref this, this t) // compiles, called as: "S e = S (a, b);" { x = 10_000; } /* this (this r) // does not compile: found 'r' when expecting ')' { // would be equivalent to: "this (S r)" x = 100_000; } this (this, this s) // does not compile: found ',' when expecting ')' { // would be equivalent to: "this (S, S s)" x = 1_000_000; } */ } import std.stdio; void main () { S a; S b = a; S c = S (a); S d = S (S (a)); S e = S (a, b); writeln (a.x, " ", a.y); // 1 0 writeln (b.x, " ", b.y); // 10 1 writeln (c.x, " ", c.y); // 100 0 writeln (d.x, " ", d.y); // 1000 0 writeln (e.x, " ", e.y); // 10000 0 } ----- Now, "this(this)" is the postblit constructor and thus an understandable exclusion. Still, the reason "this(this r)" or "this(this, this s)" do not currently compile - while other examples compile and run smoothly - seems arbitrary and technical. To always require using "typeof(this)" instead of "this" for the type would be less confusing. An intermediate yet not-so-confusing solution would be to forbid using "this" in constructor parameter declarations while still permitting it in other method declarations.

Discussion which led to this issue report: http://forum.dlang.org/thread/[email protected]

Hmm, this is caused by issue 2540. Makes me think 2540 was a mistake, as `alias x = this.foo;` is technically aliasing an expression. CCing Kenji because he was involved in the discussion.

Looks like Walter is against changing anything here: https://issues.dlang.org/show_bug.cgi?id=12228#c4

Although this was the earlier bug, I'm marking this as a duplicate: Issue 18228 covers `this` in a parameter list and is now fixed. Issue 12228 is still outstanding. *** This issue has been marked as a duplicate of issue 18228 ***