Bug 20158 – Erroneous incompatible types error for classes and interfaces when the class is not directly convertible to the interface but is convertible to a parent of the interface

Demonstration: https://run.dlang.io/is/tlyu7Z --- interface I { void foo(); } interface J : I { void bar(); } class C1 : I { override void foo() {} } class C2 : J { override void foo() {}; override void bar() {}; } I newI() { return new C1(); } J newJ() { return new C2(); } C1 newC1() { return new C1(); } void main(string[] args) { const c = args.length > 1; I blah; // Compiles: if (c) blah = newJ(); else blah = newC1(); // Fails to compile: blah = c ? newJ() : newC1(); // Error: incompatible types for (newJ()) : (newC1()): J and C1 // Fails to compile: blah = () { if (c) return newJ(); else return newC1(); // Error: mismatched function return type inference of C1 and J }(); // P.S. // When the class directly implements the interface there is no problem. auto _ = c ? newI() : newC1(); } ---

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