Bug 21243 – Allow lambdas to return auto ref

Nested functions (to which lambdas are lowered) already support this, so lambdas should too. This will make it possible to, for example, enclose an arbitrary expression in a @trusted lambda without needing to check first whether the expression evaluates to an lvalue or an rvalue.

Example per Paul: void main() { alias f1(alias x) = () => x; int n = 123; assert(__traits(compiles, f!123())); // ok assert(__traits(compiles, f!n() = 456)); // no good } If we add ref to the return value of the lambda, the other assertion fails.

There is another example use-case in issue 16034. Currently, `isInputRange` always evaluates to `false`, for ranges with non-copyable elements, because it checks for the presence of `.front` by attempting to compile the following lambda: (R r) => r.front If `r.front` returns by `ref`, this will cause its return value to be copied, which in turn causes compilation to fail when its type is non-copyable. With `auto ref` lambdas, this could be fixed by changing the lambda to auto ref (R r) => r.front As-is, the fix will require a more invasive change.

dlang/dmd pull request #14454 "Fix issue 21243 - Allow lambdas to return auto ref" was merged into master: - fac01cc29140a15f2f17cd146dc8bf507e4f57fe by Paul Backus: Fix issue 21243 - Allow lambdas to return auto ref https://github.com/dlang/dmd/pull/14454