June 02, 2023 [Issue 23955] New: Can't access non-eponymous members in IFTI template | ||||
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https://issues.dlang.org/show_bug.cgi?id=23955 Issue ID: 23955 Summary: Can't access non-eponymous members in IFTI template Product: D Version: D2 Hardware: All OS: All Status: NEW Keywords: rejects-valid Severity: normal Priority: P3 Component: dmd Assignee: nobody@puremagic.com Reporter: dlang-bugzilla@thecybershadow.net ////// test.d ////// template foo() { alias T = int; void foo(T v) { } } void main() { foo!()(1); // OK foo (1); // NG } //////////////////// Compiler says: test.d(5): Error: undefined identifier `T` test.d(13): Error: template `test.foo` is not callable using argument types `!()(int)` test.d(1): Candidate is: `foo()(T v)` I think this is an effect of how IFTI resolution is done (https://dlang.org/spec/template.html#ifti). However, - This limitation is not explicitly documented. - The error message is misleading. - This particular case should work. The argument does not have any provenance from the template parameters (of which there are none). In practice, this limitation makes "This Parameters" (https://dlang.org/spec/template.html#template_this_parameter) a lot less useful, as it's not possible to use the constness of "this" or something like typeof(This.field) in the function signature. Consider for example: struct S { int[] arr; void getArr(scope void delegate(int[]) fn) { fn(arr); } } Now, we want to make getArr callable with a const S. Attempt #1, add a const overload: void getArr(scope void delegate(int[]) fn) { fn(arr); } void getArr(scope void delegate(const(int)[]) fn) const { fn(arr); } Oops, now calling getArr on a mutable object with a lambda causes "matches both" errors. OK, no overloads. Attempt #2, let's try inout: void getArr(scope void delegate(inout(int)[]) fn) inout { fn(arr); } Oops, that doesn't actually work, inout is not smart enough to propagate to delegates. The inout on the delegate parameter always resolves to const. OK, no inout. Attempt #3, let's try This parameters: template getArr(this This) { void getArr(scope void delegate(typeof(This.arr)) fn) { fn(arr); } } Oops, that doesn't work because of the bug described here. Attempt #4, just use typeof(this.arr) instead: template getArr(this This) { void getArr(scope void delegate(typeof(this.arr)) fn) { fn(arr); } } Oops, "this" in the parameter list does not yet have the qualifiers applied to it, so it's always mutable. SOL! -- |
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