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Don't error with "used before assigned" when declared type includes void #52054

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Don't error with "used before assigned" when declared type includes void #52054

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8 changes: 6 additions & 2 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -19541,10 +19541,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return false;
}

function containsUndefinedType(type: Type) {
function containsUndefinedType(type: Type): boolean {
return !!((type.flags & TypeFlags.Union ? (type as UnionType).types[0] : type).flags & TypeFlags.Undefined);
}

function containsVoidType(type: Type): boolean {
return type.flags & TypeFlags.Union ? containsType((type as UnionType).types, voidType) : type === voidType;
}

function isStringIndexSignatureOnlyType(type: Type): boolean {
return type.flags & TypeFlags.Object && !isGenericMappedType(type) && getPropertiesOfType(type).length === 0 && getIndexInfosOfType(type).length === 1 && !!getIndexInfoOfType(type, stringType) ||
type.flags & TypeFlags.UnionOrIntersection && every((type as UnionOrIntersectionType).types, isStringIndexSignatureOnlyType) ||
Expand Down Expand Up @@ -27313,7 +27317,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return convertAutoToAny(flowType);
}
}
else if (!assumeInitialized && !containsUndefinedType(type) && containsUndefinedType(flowType)) {
else if (!assumeInitialized && !containsUndefinedType(type) && !containsVoidType(type) && containsUndefinedType(flowType)) {
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@Andarist Andarist Dec 30, 2022 •
edited
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The problem is that the typeof expression is the antecedent of the value variable here and it changes the void type somewhat unexpectedly.

narrowTypeByLiteralExpression discards void from string | void type for the assumeTrue === false case here.

So when we get to this line here we have:

type // string | void
flowType // string | undefined

As the other antecedent pushes undefined into the antecedentTypes in the getTypeAtFlowBranchLabel.

I don't think this can be changed since this is how TS behaves today:

declare const a: string | void

if (typeof a === 'undefined') {
  a // undefined
}

if (typeof a !== 'undefined') {
  a // string
}

So checking against voidType here was my best shot at fixing this.

EDIT: Alternatively... maybe the antecedents are set incorrectly here. I don't quite understand why value is "linked" to this binary expression. I didn't dive into CFA implementation too much yet so I didn't even attempt to explore the possibilities there

error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
// Return the declared type to reduce follow-on errors
return type;
Expand Down
23 changes: 23 additions & 0 deletions tests/baselines/reference/controlFlowVoidNoUsedBeforeAssigned.symbols
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@@ -0,0 +1,23 @@
=== tests/cases/conformance/controlFlow/controlFlowVoidNoUsedBeforeAssigned.ts ===
// repro #52003

declare const fn: () => string | void;
>fn : Symbol(fn, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 2, 13))

const value = fn();
>value : Symbol(value, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 3, 5))
>fn : Symbol(fn, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 2, 13))

const result = {
>result : Symbol(result, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 5, 5))

type: typeof value === "undefined" ? "A" : "B",
>type : Symbol(type, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 5, 16))
>value : Symbol(value, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 3, 5))

value: value,
>value : Symbol(value, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 6, 51))
>value : Symbol(value, Decl(controlFlowVoidNoUsedBeforeAssigned.ts, 3, 5))

};

31 changes: 31 additions & 0 deletions tests/baselines/reference/controlFlowVoidNoUsedBeforeAssigned.types
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@@ -0,0 +1,31 @@
=== tests/cases/conformance/controlFlow/controlFlowVoidNoUsedBeforeAssigned.ts ===
// repro #52003

declare const fn: () => string | void;
>fn : () => string | void

const value = fn();
>value : string | void
>fn() : string | void
>fn : () => string | void

const result = {
>result : { type: string; value: string | undefined; }
>{ type: typeof value === "undefined" ? "A" : "B", value: value,} : { type: string; value: string | undefined; }

type: typeof value === "undefined" ? "A" : "B",
>type : string
>typeof value === "undefined" ? "A" : "B" : "A" | "B"
>typeof value === "undefined" : boolean
>typeof value : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
>value : string | void
>"undefined" : "undefined"
>"A" : "A"
>"B" : "B"

value: value,
>value : string | undefined
>value : string | undefined

};

12 changes: 12 additions & 0 deletions tests/cases/conformance/controlFlow/controlFlowVoidNoUsedBeforeAssigned.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
// @strict: true
// @noEmit: true

// repro #52003

declare const fn: () => string | void;
const value = fn();

const result = {
type: typeof value === "undefined" ? "A" : "B",
value: value,
};