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Return early on intrinsics in couldContainTypeVariables #54377

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Return early on intrinsics in couldContainTypeVariables #54377

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1 change: 1 addition & 0 deletions src/compiler/checker.ts
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Expand Up @@ -24004,6 +24004,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// we perform type inference (i.e. a type parameter of a generic function). We cache
// results for union and intersection types for performance reasons.
function couldContainTypeVariables(type: Type): boolean {
if (type.flags & TypeFlags.Intrinsic) return false;
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@ahejlsberg ahejlsberg May 25, 2023

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I'm not understanding how the function would previously return true for the wildcard type (which is a TypeFlags.Any)? What am I missing?

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@jakebailey jakebailey May 25, 2023

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The place I was referring to was inferTypes, which starts with a wildcard check, but if both sides are a wildcard we loop forever.

But, it checks for "could contain type var" before the wildcard logic.

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@ahejlsberg ahejlsberg May 25, 2023

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Right, but assuming both sides are wildcard types, how is it that we loop forever? That would require couldContainTypeVariables to return true for a TypeFlags.Any, and I'm not seeing how that happens.

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@jakebailey jakebailey May 25, 2023

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You're totally right that couldContainTypeVariables should already be returning false for wildcardType, and yet, in my example it appeared to be returning true in the debugger!

But now, on main, my test doesn't fail... reverse bisecting says that it was "fixed" by changing lib.d.ts. So, I think there's some really odd corruption happening. Sounds like a fun debugging challenge.

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@jakebailey jakebailey Jun 2, 2023

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Just to note the scary thing I'm seeing:

image

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@jakebailey jakebailey Jun 2, 2023

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Traced this down to the new code in #53246, but it's absolutely bizarre. The flag is clearly there that says that "could contain type vars" is computed, and yet it still proceeds to overwrite it for no reason.

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@jakebailey jakebailey Jun 2, 2023

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Figured it out, see #54348 (comment)

const objectFlags = getObjectFlags(type);
if (objectFlags & ObjectFlags.CouldContainTypeVariablesComputed) {
return !!(objectFlags & ObjectFlags.CouldContainTypeVariables);
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