[Issue 24030] New: A `lazy` parameter shouldn't be allowed to be "called" twice
https://issues.dlang.org/show_bug.cgi?id=24030
Issue ID: 24030
Summary: A `lazy` parameter shouldn't be allowed to be "called"
twice
Product: D
Version: D2
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P1
Component: dmd
Assignee: nob...@puremagic.com
Reporter: qs.il.paperi...@gmail.com
It’s quite straightforward: The intended use for `lazy` is to defer evaluation;
everyone seeing this for the first time expects that. The fact that a function
taking a `lazy` parameter may evaluate the underlying delegate more than once
is surprising. Only once a programmer learns that there is a delegate
underlying, it makes some sense.
This is a breaking change, but there’s a simple transition path: Use a delegate
explicitly, and you’ll surprise nobody.
Checking that a `lazy` parameter is evaluated at most once only needs a very
limited form of control-flow analysis. In `@system` code, this could be ignored
and calling a `lazy` parameter more than once becomes Undefined Behavior.
If desired, on non-release builds, the only-call-once rule can be asserted by
introducing a hidden `bool` variable on the caller’s side:
```d
int f(lazy int);
1 + f(1);
```
is lowered to:
```d
int f(int delegate());
bool called; // unique name
1 + f({
assert(!called);
called = true;
return 1;
});
```
What about `lazy` `void` parameters?
They’re no different. You’re after the side-effect, but again, getting it more
than once will be surprising; use a delegate.
--
[Issue 24024] cannot pass class this to ref class
https://issues.dlang.org/show_bug.cgi?id=24024 kinke changed: What|Removed |Added CC||ki...@gmx.net --- Comment #1 from kinke --- This is the behavior I would expect - a class-ref `this` is special and not to be mutated. What if `foo()` sets it to null? --
[Issue 24025] Expressions contained in parentheses should not be assumed to be C casts
https://issues.dlang.org/show_bug.cgi?id=24025 Dlang Bot changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED --- Comment #3 from Dlang Bot --- dlang/dmd pull request #15377 "Fix Issue 24025 - Expressions contained in parentheses should not be …" was merged into master: - 9f782c456e31a4fad5027961d64c346d388f1b7e by Nick Treleaven: Fix Issue 24025 - Expressions contained in parentheses should not be assumed to be C casts Allow `(identifier)` and `(BasicType)` when followed by a left parenthesis. These can be a function call or an implicit conversion - the latter may produce a semantic error (not a parse error). https://github.com/dlang/dmd/pull/15377 --
[Issue 24029] ImportC: symbol name clash on statement expressions
https://issues.dlang.org/show_bug.cgi?id=24029 Dlang Bot changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED --- Comment #2 from Dlang Bot --- dlang/dmd pull request #15378 "fix Issue 24029 - ImportC: symbol name clash on statement expressions" was merged into master: - aa7024012d1d196dbf36b49f0e2982dcb01d247e by Walter Bright: fix Issue 24029 - ImportC: symbol name clash on statement expressions https://github.com/dlang/dmd/pull/15378 --
[Issue 23966] [REG2.102] Cannot use traits(getAttributes) with overloaded template
https://issues.dlang.org/show_bug.cgi?id=23966 Dlang Bot changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED --- Comment #5 from Dlang Bot --- dlang/dmd pull request #15353 "Fix Issue 23966 - [REG2.102] Cannot use traits(getAttributes) with overloaded template" was merged into stable: - bf2c04df822eee078ec0debb151c09d794f709c9 by RazvanN7: Fix Issue 23966 - [REG2.102] Cannot use traits(getAttributes) with overloaded template https://github.com/dlang/dmd/pull/15353 --
