Allow narrowing {} originating in unknown using instanceof
#51014
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src/compiler/checker.ts
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| if (!nonConstructorTypeInUnion) return type; | ||
| } | ||
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| if (assumeTrue && declaredType === unknownType && isEmptyAnonymousObjectType(type)) { |
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Perhaps this condition should be relaxed somehow or maybe it should be pushed further down the road to getNarrowedType. This is the version that I'm most confident with though ;p
The added check is roughly based on the check recently introduced here: https://github.com/microsoft/TypeScript/pull/50610/files#diff-d9ab6589e714c71e657f601cf30ff51dfc607fc98419bf72e04f6b0fa92cc4b8R25153
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| // repro #51007 | ||
| function isHTMLTable(table: unknown): boolean { | ||
| return !!table && table instanceof Object && 'html' in table; |
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I think the test here should actually test for an access of table.html in some capacity - e.g.
function tryGetHtmlPropFromTable(table: unknown) {
if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
return table.html;
}
return undefined;
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hm, what would it change here? the reported error was that in couldn't be used at all in this situation - we could add more "assertions" here than what was originally reported but I'm not sure what this particular snippet would bring to the table here. What kind of nuance am I missing? 😅
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The original test is fine; having another test that actually tries to narrow and use the newly-tested property corresponds to something closer to real-world usage.
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Ok, I've pushed out the proposed test case - please take a look if this is what you had in mind here.
# Conflicts: # src/compiler/checker.ts
| if (assumeTrue && declaredType === unknownType && isEmptyAnonymousObjectType(type)) { | ||
| return targetType; | ||
| } | ||
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The problem with conditioning this on the declared type is that it doesn't work when the empty anonymous object type originates in some other type. For example:
function f<T>(x: T & {}) {
return x instanceof Object && 'a' in x; // Error, but shouldn't be
}
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Closing as superseded by #51631 |
4 participants
fixes #51007