Detect impossible equalities

[ochafik]

Strong-mode could / should prevent accidental heterogeneous (in)equalities "when it can":

var x = "1";
if (x == 2) print("Hey");
//  ^
// [error]: A 'String' instance can never be equal to a 'num'.

Of course, this is only possible (accurate) for classes that can't be implemented by user classes (String, num, etc):

class Foo {}
class Bar {}
class FooBar implements Foo, Bar {}

Foo foo = ...;
Bar bar = ...;
if (foo == bar) // Foo and Bar aren't related, yet this can be true because of FooBar

[mezoni]

Your judgements are wrong.
This is correct operation and it allowed.
Nothing wrong, everything is well in your case.

bool operator ==(Object other) 

https://github.com/dart-lang/sdk/blob/master/runtime/lib/string_patch.dart#L249

P.S.
Such a tool like a lint should detect such a cases.
But it is not a part of the SDK.

[eernstg]

You might want to take a look at 'Related Types' (
https://books.google.dk/books?id=-7OrCAAAQBAJ&pg=PA434&lpg=PA434) where
Johnni Winther describes a number of similar considerations in the context
of Java.

The point is that it is unlikely (if not impossible) that a comparison for
equality among objects of different types will produce the result true,
and if your code contains an if whose condition is always false (even
though that's not necessarily so easy to see) then your program is probably
buggy. If you really want to force such a comparison then you can cast the
operands to a common supertype.

But it will almost never be guaranteed that the comparison yields false,
because anyone can write a class that implements arbitrary other types
(except a few built-in ones like int and String), so it's more
reasonable to make it a lint than a warning or even an error.

On Thu, Apr 21, 2016 at 2:11 PM, mezoni notifications@github.com wrote:

Your judgements are wrong.
This is correct operation and it allowed.
Nothing wrong, everything is well in your case.

bool operator ==(Object other)

https://github.com/dart-lang/sdk/blob/master/runtime/lib/string_patch.dart#L249

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#26311 (comment)

Erik Ernst - Google Danmark ApS
Skt Petri Passage 5, 2 sal, 1165 København K, Denmark
CVR no. 28866984

[ochafik]

@eernstg thanks for the reference!

then your program is probably buggy

That's the point of this proposal: detect buggy programs :-)

it will almost never be guaranteed that the comparison yields false,
because anyone can write a class that implements arbitrary other types
(except a few built-in ones like int and String)

Agreed, it's precisely what I suggested: "when it can" (int, bool and String are frequent enough to catch misuses, imho).

[ochafik]

Note: TypeScript detects these cases (cc/ @leafpetersen)

[zoechi]

This seems related dart-archive/linter#229

[mezoni]

@ochafik

Note: TypeScript detects these cases (cc/ @leafpetersen)

This is incorrect comparison because in Dart language everything is an instance of the Object but in Typescript this is not true.

And I agree that this is an illogical action (which maybe a bug) but analyzer is not a tool which should used for detecting illogical actions in the source code, for such purpose should be used other tools.

[eernstg]

Surely, the analyzer's main job is to track specified error and warning
situations, but I don't think it is unreasonable to give a hint when
objects of unrelated types are compared for equality. So it doesn't have to
be a different tool.

On Thu, Apr 28, 2016 at 2:57 PM, mezoni notifications@github.com wrote:

@ochafik https://github.com/ochafik

Note: TypeScript detects these cases (cc/ @leafpetersen
https://github.com/leafpetersen)

This is incorrect comparison because in Dart language everything is an
instance of the Object but in Typescript this is not true.

And I agree that this is an illogical action (which maybe a bug) but
analyzer is not a tool which should used for detecting illogical actions in
the source code, for such purpose should be used other tools.

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#26311 (comment)

Erik Ernst - Google Danmark ApS
Skt Petri Passage 5, 2 sal, 1165 København K, Denmark
CVR no. 28866984

[alexeieleusis]

Are there any cases besides the ones covered in dart-archive/linter#232 that we want to implement?

[ochafik]

@alexeieleusis Not that I can think of, good work / thanks for adding that lint!

Closing (unless anyone feels strongly this should make it to strong mode?)