Discriminate contextual types #19733
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sandersn
requested changes
Nov 6, 2017
src/compiler/checker.ts
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| return type && getApparentType(type); | ||
| let contextualType = getContextualType(node); | ||
| contextualType = contextualType && getApparentType(contextualType); | ||
| if (contextualType && contextualType.flags & TypeFlags.Union && isObjectLiteralExpression(node)) { |
src/compiler/checker.ts
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| contextualType = contextualType && getApparentType(contextualType); | ||
| if (contextualType && contextualType.flags & TypeFlags.Union && isObjectLiteralExpression(node)) { | ||
| let match: Type | undefined; | ||
| propLoop: for (const prop of node.properties) { |
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this code is almost exactly the same as the also-highly-questionable findMatchingDiscriminantType. The only real difference, I suspect, is that this code doesn't bail if (type === match). Two things:
- Can you harmonise the two code paths so there's only one
findMatchingDiscriminantType?
2.It's quite possible that the excess-property usage and union-error usage would both benefit from the(type === match)addition, so it could be that you can just swap out the existing body for this code. - Three things! Using
findMatchingDiscriminantTypeis slow, and only makes the compiler faster because it eliminates unions early on. Can you test performance with this change?
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findMatchingDiscriminantTypeoperates on type members (andfindMatchingDiscriminantTypedirectly usesisRelatedTo), this operates on an actual node tree (as most contextual type operations do), so sadly I don't think they can be merged (without a bunch of inefficient abstractions over weather you're getting symbols from types or nodes and such)- Sure, I'll try modifying
findMatchingDiscriminantType, too; but I don't think it'll show up in much unless we add a test case where multiple fields are capable of acting as a discriminant for an object and discriminate to the same types. 🐱 - Sure, will do.
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Oh hey, waddoyaknow, we already do have a test (excessPropertyCheckWithUnions) improved by changing findMatchingDiscriminantType. Neat.
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@sandersn As far as overall perf goes, this has no real effect on any codebase in our perf suite:
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sandersn
approved these changes
Nov 6, 2017
ghost
mentioned this pull request
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This was split from #19587, and the idea originated from a comment on #19322 (and then the code moved directly into
getApparentTypeOfContextualTypeinstead ofgetContextualThisParameterTypeso as to be more general), and should be merged after #19587. The primary baseline changes will be visible in tests added in that PR; however there are some small changes already visible here because we toss out uninteresting contextual types earlier based on members we know the type of.This causes a few things:
thistypes are discriminated based on the other members you wrote; meaning methods in objects contextually typed by something now have thethistype of only the discriminated member you've indicated, if possible. (This is the primary benefit the tests in the other PR can show)