[ hh_client --lint ] False positive for 5607, equality checks a generic value to a typed value is not always false #8940
Description
Describe the bug
Given the expression $expression_resolving_to_generic_type === $expression_resolving_to_a_known_type, lint rule 5607 should not emit an error if the generic type has any overlap with the known type.
Standalone code, or other way to reproduce the problem
function is_life<T>(T $t): (bool, T) {
return tuple($t === 42, $t);
}Steps to reproduce the behavior:
hh_client --lint file.hack
Expected behavior
No linter errors! Because T does not have a constraint that says it can not be an int, therefore it can be an int. $t === 42 is not always false.
Actual behavior
Lint[5607] Invalid comparison: This expression will always return false.
A value of type T can never be equal to a value of type int [1]
file.hack:2:16
1 | function is_life<T>(T $t): (bool, T) {
[1] 2 | return tuple($t === 42, $t);
3 | }
Environment
- Operating system
Ubuntu 20.04.
- Installation method
apt-get with dl.hhvm.com repository
- HHVM Version
Please include the output of
hhvm --versionandhh_client --version
HipHop VM 4.137.0 (rel) (non-lowptr)
Compiler: 1637698090_520463089
Repo schema: d90319777b5acb5402a80af5652b95f8dcdd4048
hackc-261ee251a4f119d21a986be5fb0c88d2e404b515-4.137.0
Additional context
This version compares ?T to ?int, which is also a false positive.
Lint[5607] Invalid comparison: This expression will always return false.
A value of type ?T can never be equal to a value of type ?int [1]
file.hack:3:16
1 | function is_life<T>(bool $unknown_to_typechecker, ?T $t): (bool, ?T) {
2 | $maybe_null = $unknown_to_typechecker ? null : 42;
[1] 3 | return tuple($t === $maybe_null, $t);
4 | }
But this false positive goes away for always-null checks.
function is_life<T>(bool $unknown_to_typechecker, ?T $t): (bool, ?T) {
$maybe_null = $unknown_to_typechecker ? null : null;
return tuple($t === $maybe_null, $t);
}
No lint errors!