It should not be possible to implement an interface twice

[DartBot]

This issue was originally filed by ief...@unipro.ru

---

What steps will reproduce the problem?
consider the following test:
interface I {}
interface J {}

class A implements I, J, I {}

main() {
  new A();
}

What is the expected output? What do you see instead?
Expected: compile-time error (It is a compile-time error if a type T appears more than once in the implements clause of a class)

Actual: successful compilation

What version of the product are you using? On what operating system?
DartVM r2810

Please provide any additional information below.
co19 test
LangSpecTest/07_Classes/9/Superinterfaces/A04/t01

[dgrove]

Added Area-VM, Triaged labels.

[DartBot]

This comment was originally written by @mhausner

---

The language spec indeed does not allow this:

It is a compile-time error if a type T appears more than once in the implements clause of a class.

One might argue that it is harmless to repeat a type in this way, so why make it an error? The issue is not so much that the situation described in program source is erroneous, but that it is pointless. [...]

How about this:

interface I {}
interface J {}

class A implements I { }
class B extends A implements I, J { }

...

Is it an error to have I in both A and B? With the above rationale, one could say it should be illegal.

---

Set owner to @mhausner.

[iposva-google]

Marked this as being blocked by #1031.

[DartBot]

This comment was originally written by @mhausner

---

This is now fixed in the VM. Frog and DartC need to fix this as well.

New test language/DuplicateImplementsTest fails for both frog and dartc.

---

Removed Area-VM label.
Added Area-Frog label.

[DartBot]

This comment was originally written by @mhausner

---

Removed the owner.
Changed the title to: "Frog: it should not be possible to implement an interface twice".

[DartBot]

This comment was originally written by ief...@unipro.ru

---

I'm not sure that this bug was correctly fixed in VM.
The spec v0.07 says:

It is a compile-time error if a type T appears more than once in the implements clause of a class. (7.9. Superinterfaces)

However, section 8.4 Superinterfaces does not prohibit duplicate types in the extends clause of an interface.

So, the following test should pass:

interface J {}
interface K {}

interface I extends J, K, J, K {}

main() {}

[anders-sandholm]

Removed Area-Frog label.
Added Area-Dart2JS, FromAreaFrog labels.
Marked this as being blocked by #1031.
Unmarked this as being blocked by #-1031.

[kasperl]

We need to figure out if this issue applies to dart2js.

---

Removed FromAreaFrog label.
Changed the title to: "It should not be possible to implement an interface twice".

[kasperl]

Added this to the Later milestone.
Marked this as being blocked by #1031.
Unmarked this as being blocked by #1031.

[kasperl]

Removed this from the Later milestone.

[kasperl]

Added this to the M2 milestone.

[peter-ahe-google]

Set owner to @karlklose.

[kasperl]

Removed this from the M2 milestone.
Added this to the M3 milestone.

[kasperl]

Removed this from the M3 milestone.
Added this to the M4 milestone.

[karlklose]

Added Fixed label.