Bug 22792 – Wrong inference of opApply

Why is following code different behavior between int and string? ----------------- import std; struct S { int opApply(int delegate(ref const(int), ref int) dg) { writeln("mutable int"); return 0; } int opApply(int delegate(ref const(int), ref const(int)) dg) const { writeln("const int"); return 0; } int opApply(int delegate(ref const(int), ref string) dg) { writeln("mutable str"); return 0; } int opApply(int delegate(ref const(int), ref const(string)) dg) const { writeln("const str"); return 0; } } void main() { S s; foreach (ref const(int) x, ref const(int) y; s) {} foreach (ref const(int) x, ref const(string) y; s) {} } ----------------- rdmd playground.d const int mutable str ----------------- The function on the const one that matches better should be called. This problem was discovered by investigating the cause of Botan(*) build failure in 2.099.1-beta.1. However, the above code shows the same behavior since before 2.098.1. * https://github.com/etcimon/botan

> The function on the const one that matches better should be called. Neither of the string opApply's is an exact match due to the const missmatch on `this` or the delegate parameter => "match with implicit conversions". Partial ordering then selects the mutable overload because it is "more specialized" (cannot be called by the const overload). Not sure why the int overloads expose a different behaviour. See https://dlang.org/spec/function.html#function-overloading

Slightly modified test case: import core.stdc.stdio; struct S { int opApply(int delegate(ref const(int), ref int) dg) { puts("mutable int"); return 0; } int opApply(int delegate(ref const(int), ref const(int)) dg) const { puts("const int"); return 0; } int opApply(int delegate(ref const(int), ref string) dg) { puts("mutable str"); return 0; } int opApply(int delegate(ref const(int), ref const(string)) dg) const { puts("const str"); return 0; } } void main() { S s; foreach (ref const(int) x, ref const(int) y; s) {} foreach (ref const(int) x, ref const(string) y; s) {} const S s2; foreach (ref const(int) x, ref const(int) y; s2) {} foreach (ref const(int) x, ref const(string) y; s2) {} } Up to 2.074.1: Success with output: ----- const int const str const int const str ----- Since 2.075.1: Success with output: ----- const int mutable str const int const str -----

Same behaviour without the first parameter that is identical for every overload: struct S { int opApply(int delegate(ref int) dg) { puts("mutable int"); return 0; } int opApply(int delegate(ref const(int)) dg) const { puts("const int"); return 0; } int opApply(int delegate(ref string) dg) { puts("mutable str"); return 0; } int opApply(int delegate(ref const(string)) dg) const { puts("const str"); return 0; } } void main() { S s; foreach (ref const(int) y; s) {} foreach (ref const(string) y; s) {} const S s2; foreach (ref const(int) y; s2) {} foreach (ref const(string) y; s2) {} }

(In reply to moonlightsentinel from comment #1) > [...] "match with implicit conversions". Correction: This applies only to the mutable overload because the delegate type changes due to the different constness. The const overload should be "match with qualifier conversion" (`S` => `const S`).

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