Fix circular mapped type instantiations for arrays and tuples #27911
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ahejlsberg requested review from DanielRosenwasser, RyanCavanaugh and sandersn 
sandersn reviewed Oct 15, 2018
| !!! error TS2322: Type '{ a: number; }' is not assignable to type '{ a?: undefined; b?: undefined; }'. | ||
| !!! error TS2322: Types of property 'a' are incompatible. | ||
| !!! error TS2322: Type 'number' is not assignable to type 'undefined'. | ||
| !!! error TS2322: Type '{ a: number; }' is not assignable to type '{ a: number; b: number; }'. |
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any idea why this changes? I don't see any mapped types here.
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Never mind, didn't see the change to discriminant types.
sandersn reviewed Oct 15, 2018
| return undefined; | ||
| } | ||
| | ||
| function isDiscriminantType(type: Type): boolean { |
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this seems unrelated to the bug fix in the PR description
sandersn reviewed Oct 15, 2018
| } | ||
| let combined = 0; | ||
| for (const t of (<UnionType>type).types) combined |= t.flags; | ||
| if (combined & TypeFlags.Unit && !(combined & TypeFlags.Instantiable)) { |
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what happens if you have two two non-unit types? Should that still be discriminable?
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I'm pretty sure this change isn't supposed to be here, since it's #27695
| I accidentally based this PR on the |
sandersn approved these changes Oct 15, 2018
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Mapped types can circularly reference themselves (because member resolution is deferred), but array and tuple type instantiations cannot. This causes an issue when a circular homomorphic mapped type is instantiated for an array or tuple type as we now create array and tuple instantiations for such mappings (see #26063). With is PR we quickly detect and stop the runaway recursion that can occur by substituting
errorTypefor the instantiation. We would eventually do this after 50 levels of nested instantiations, but exponential recursive fan-out could keep us from ever getting there.Fixes #27881.