Clearer error message when a class extends an interface

[MatrixFrog]

https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%2520%2540interface%2520*%252F%250Aclass%2520MyInterface%2520%257B%257D%250A%250Aclass%2520MyClass%2520extends%2520MyInterface%2520%257B%257D%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26CHECK_TYPES%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1%26TRANSPILE%3D1

Currently this gives two different errors, and one of them mentions $jscomp.inherits which is totally unhelpful and meaningless to the user. We should:

1. Only report one error
2. Make sure it's clear
3. (bonus) Suggest that the user probably meant to include an @implements annotation in their JSDoc, rather than writing extends

[MatrixFrog]

@Dominator008 want to take a stab at this?

[Dominator008]

input0:4: WARNING -  cannot extend this type; constructors can only extend constructors
class MyClass extends MyInterface {}
^

input0:4: WARNING - actual parameter 2 of $jscomp.inherits does not match formal parameter
found   : function (this:MyInterface): ?
required: Function
class MyClass extends MyInterface {}
^

0 error(s), 2 warning(s), 89.8% typed

I think both of them come from TypeCheck, which knows nothing about ES6. If we want to generate a meaningful and helpful error message from the transpilation pass would it be a third error (or first, in output order)?

[MatrixFrog]

Or the transpilation could add a little bit of metadata saying "this used
to be an ES6 class" which the typechecker could use to modify its error
messages. That could get unwieldy though. I'm not sure.

The first message is clearer IMO so just getting rid of the second somehow
would help quite a bit.
On Mar 11, 2016 4:34 PM, "Michael Zhou" notifications@github.com wrote:

input0:4: WARNING - cannot extend this type; constructors can only extend constructors
class MyClass extends MyInterface {}
^

input0:4: WARNING - actual parameter 2 of $jscomp.inherits does not match formal parameter
found : function (this:MyInterface): ?
required: Function
class MyClass extends MyInterface {}
^

0 error(s), 2 warning(s), 89.8% typed

I think both of them come from TypeCheck, which knows nothing about ES6.
If we want to generate a meaningful and helpful error message from the
transpilation pass would it be a third error (or first, in output order)?

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#1643 (comment)
.

[blickly]

I don't think we want to complicate typechecking just to give a better message here.

@MatrixFrog Is the unqualified name the main case that we care about? If so, it might be easy to write a syntactic check based just on the @interface annotation in the JSDoc.

[MatrixFrog]

The problem is the interface might be in another file and we want transpilation to work in a file-by-file way.

[Dominator008]

@MatrixFrog @blickly This is not ES6 / inheritance related. Smaller repro:

/** @interface */ function Foo() {};
var /** Function */ x = Foo;

https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%2520%2540interface%2520*%252F%2520function%2520Foo()%2520%257B%257D%253B%250A%250Avar%2520%252F**%2520Function%2520*%252F%2520x%2520%253D%2520Foo%253B%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_TYPES%3D1