DDC: Evaluation of e1 == e2 should detect null before any type checks

[eernstg]

Consider the following program (which is a slice of language/operator/equality_covariant_test.dart, which is added by https://dart-review.googlesource.com/c/sdk/+/145582):

import "package:expect/expect.dart";

class EqNever {
  operator ==(covariant Never other) => throw "unreachable";
}

void main() {
  Object oNever = EqNever();
  Object? myNull = null;
  Expect.isFalse(oNever == myNull);
}

This test fails on the configuration ddc-nnbd-linux-release-chrome-try (log), with the following stack trace:

Deobfuscated error and stack:
Error: Expected a value of type 'Never', but got one of type 'Null'
    at Object.throw_                               out/ReleaseX64/gen/utils/dartdevc/kernel/amd/dart_sdk.js 4029:11
    at Object.castError                            ../../../../../dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 62:3
    at Object.cast                                 ../../../../../dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 442:10
    at dart.NeverType.new.as                       ../../../../../dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart 82:19
    at equality_covariant_test.EqNever.new._equals out/ReleaseX64/generated_compilations/dartdevk-checked-sdk/tests_language_operator_equality_covariant_test/equality_covariant_test.js 15:18
    at Object.main                                 ../../../../../tests/language/operator/equality_covariant_test.dart 36:24
    at testMainWrapper                             out/ReleaseX64/generated_tests/dartdevk-chrome-checked-sdk/tests_language_operator_equality_covariant_test/test.html 87:93

However, evaluation of an expression of the form e1 == e2 is specified to run in phases where null is processed first. In particular, oNever == myNull should evaluate oNever to an object different from null, and evaluate myNull to null, and then yield false, and up to this point there is no type check of any of these objects.

The actual behavior is that it is checked whether null is of type Never, which is not the case in strong checking mode, and the evaluation then throws rather than yielding false.

[nshahan]

Just to double check, is this an issue when testing equality with an object that has a runtime type of Null specifically? If you were perform the same equality check with an integer instead of null:

...
Object? myInt = 42;
Expect.isFalse(oNever == myInt);

Would throwing a TypeError be expected?

[eernstg]

Yes. The only object that has a run-time type of Null is the null object, so any rules about the latter are also rules about the former. (It is possible to override runtimeType, so in that sense any object could have a run-time type of Null, but that won't change the outcome of language defined run-time type queries).

With oNever == myInt the special treatment of null is irrelevant (none of the operands is the null object), so we proceed to invoke operator == on oNever, which will throw a TypeError.