tests/cases/conformance/interfaces/declarationMerging/mergedInterfacesWithMultipleBases4.ts(19,11): error TS2320: Interface 'A' cannot simultaneously extend types 'C' and 'C'. Named properties 'a' of types 'C' and 'C' are not identical. ==== tests/cases/conformance/interfaces/declarationMerging/mergedInterfacesWithMultipleBases4.ts (1 errors) ==== // merged interfaces behave as if all extends clauses from each declaration are merged together class C { a: T; } class C2 { b: T; } class C3 { c: T; } class C4 { d: T; } interface A extends C, C3 { // error ~ !!! error TS2320: Interface 'A' cannot simultaneously extend types 'C' and 'C'. !!! error TS2320: Named properties 'a' of types 'C' and 'C' are not identical. y: T; } interface A extends C, C4 { z: T; } class D implements A { a: string; b: string; c: string; d: string; y: boolean; z: boolean; }