microsoft / microsoft/TypeScript
Can't evaluate equality of symbols made by Symbol.for
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描述
This is also an obstacle of branded types and ghost types. I think resolving this problem is easy because these types are just a new literal type similar to string literal types.
TypeScript Version: 3.7.x-dev.20191228
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Code
const a = Symbol.for('');
const b = Symbol.for('');
const c: typeof b = Symbol.for(''); // error
a === b; // error
Expected behavior:
no error
Actual behavior:
Type 'typeof b' is not assignable to type 'typeof a'.
This condition will always return 'false' since the types 'typeof a' and 'typeof b' have no overlap.
Playground Link: https://www.typescriptlang.org/play/index.html?ts=3.8.0-dev.20191228&ssl=1&ssc=1&pln=4&pc=18#code/MYewdgzgLgBAhjAvDAygTwLYCMQBsB0AZiAE4AUA5BQJQDcAUKJLFkqpjgceVXY+NBjAAXDChoADgFMQhGK2TpseIqUo1aMAPRaYUkiVL0EiU-M069B0kA
Related Issues: #35200
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调研方向
Start by reproducing the Symbol.for example in the linked Playground or with the reported TypeScript version, including the assignment and equality errors. Trace how the type checker treats separately declared symbols and compare the behavior with the related issue #35200; done means the shown code produces no errors while preserving symbol type checking.
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