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Limit inference from apparent types to one level deep #27225

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Merged
weswigham merged 3 commits into from
Sep 20, 2018
Merged

Limit inference from apparent types to one level deep #27225

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264b75b
Limit inference from apparent types to one level deep
weswigham Sep 19, 2018
d372f2c
Rename marker & use booleanness not ===
weswigham Sep 19, 2018
26295be
Undo typo
weswigham Sep 19, 2018
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11 changes: 10 additions & 1 deletion src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -13439,6 +13439,7 @@ namespace ts {
let visited: Map<boolean>;
let contravariant = false;
let propagationType: Type;
let allowComplexConstraintInference = true;
inferFromTypes(originalSource, originalTarget);

function inferFromTypes(source: Type, target: Type): void {
Expand Down Expand Up @@ -13616,7 +13617,15 @@ namespace ts {
// getApparentType can return _any_ type, since an indexed access or conditional may simplify to any other type.
// If that occurs and it doesn't simplify to an object or intersection, we'll need to restart `inferFromTypes`
// with the simplified source.
if (apparentSource !== source && !(apparentSource.flags & (TypeFlags.Object | TypeFlags.Intersection))) {
if (apparentSource !== source && allowComplexConstraintInference && !(apparentSource.flags & (TypeFlags.Object | TypeFlags.Intersection))) {
// TODO: The `allowComplexConstraintInference` flag is a hack! This forbids inference from complex constraints within constraints!
// This isn't required algorithmically, but rather is used to lower the memory burden caused by performing inference
// that is _too good_ in projects with complicated constraints (eg, fp-ts). In such cases, if we did not limit ourselves
// here, we might produce more valid inferences for types, causing us to do more checks and perform more instantiations
// (in addition to the extra stack depth here) which, in turn, can push the already close process over its limit.
// TL;DR: If we ever become generally more memory efficienct (or our resource budget ever increases), we should just
// remove this `allowComplexConstraintInference` flag.
allowComplexConstraintInference = false;
return inferFromTypes(apparentSource, target);
}
source = apparentSource;
Expand Down