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A second attempt at readonly array support persisting through isArray #42316

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A second attempt at readonly array support persisting through isArray #42316

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2 changes: 1 addition & 1 deletion lib/lib.es5.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1408,7 +1408,7 @@ interface ArrayConstructor {
(arrayLength?: number): any[];
<T>(arrayLength: number): T[];
<T>(...items: T[]): T[];
isArray(arg: any): arg is any[];
isArray(arg: any): arg is readonly unknown[];
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This is a safer type, but it can't help but break anybody who was previously able to mutate an array or use its contents after checking it with isArray. I think it's too breaky for the benefit.

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@sandersn You mean anybody who was previously able to mutate a mutable array after checking it with isArray? I tested this change locally and confirmed it doesn't break that: Mutable arrays remain mutable. The type predicate narrows the type to the intersection of typeof arg & readonly unknown[], which is only immutable if arg was already immutable.

Would you consider reopening this PR?

⏯️ Playground link

Workbench repro.

🧑‍💻 Code

declare const mutable: string[];
declare const immutable: readonly string[];

// Mutable arrays remain so with and without
// https://github.com/microsoft/TypeScript/pull/42316
if (Array.isArray(mutable)) {
  const should: string[] = mutable; // ✔️ A mutable array should remain so
}

// This assignment should error but doesn't, not without
// https://github.com/microsoft/TypeScript/pull/42316
if (Array.isArray(immutable)) {
  const shouldNot: string[] = immutable; // ❌ An immutable array should remain so
}

🙁 Actual behavior

$ tsc input.ts

No errors: You're free to mutate an immutable array.

🙂 Expected behavior

With this PR:

$ node built/local/tsc.js input.ts 
input.ts:13:9 - error TS4104: The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

13   const shouldNot: string[] = immutable; // ❌ An immutable array should remain so
           ~~~~~~~~~


Found 1 error in input.ts:13

Mutating an immutable array is an error.

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What I mean is that if somebody writes this today, it's not an error:

declare const u: unknown;
if (Array.isArray(u)) {
    u[1] = 12
    u[1].length
}

unknown narrowing is an important scenario for Array.isArray, and this change would add errors that were not there before and are not necessarily correct.

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@sandersn I should've realized what you meant, thanks for your patience.

Instead of replacing the existing signature, would adding overloads for ArrayLike<T> and Iterable<T> work, without affecting the current behavior of any and unknown? I've opened #48228 with that modification to this PR.

readonly prototype: any[];
}

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15 changes: 10 additions & 5 deletions src/compiler/checker.ts
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Expand Up @@ -22999,11 +22999,16 @@ namespace ts {
}
}

// If the candidate type is a subtype of the target type, narrow to the candidate type,
// if the target type is a subtype of the candidate type, narrow to the target type,
// otherwise, narrow to an intersection of the two types.
return isTypeSubtypeOf(candidate, type) ? candidate : isTypeSubtypeOf(type, candidate) ? type : getIntersectionType([type, candidate]);
}
// If the candidate type is a subtype of the target type, narrow to the candidate type.
// Otherwise, if the target type is assignable to the candidate type, keep the target type.
// Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
// type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
// two types.
return isTypeSubtypeOf(candidate, type) ? candidate :
isTypeAssignableTo(type, candidate) ? type :
isTypeAssignableTo(candidate, type) ? candidate :
getIntersectionType([type, candidate]);
}

function narrowTypeByCallExpression(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
if (hasMatchingArgument(callExpression, reference)) {
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8 changes: 4 additions & 4 deletions tests/baselines/reference/instanceOfAssignability.types
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Expand Up @@ -70,8 +70,8 @@ function fn2(x: Base) {
// 1.5: y: Base
// Want: y: Derived1
let y = x;
>y : Base & Derived1
>x : Base & Derived1
>y : Derived1
>x : Derived1
}
}

Expand Down Expand Up @@ -104,8 +104,8 @@ function fn4(x: Base|Derived2) {
// 1.5: y: {}
// Want: Derived1
let y = x;
>y : (Base | Derived2) & Derived1
>x : (Base | Derived2) & Derived1
>y : Derived1
>x : Derived1
}
}

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16 changes: 8 additions & 8 deletions tests/baselines/reference/typeGuardIntersectionTypes.symbols
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Expand Up @@ -176,37 +176,37 @@ function identifyBeast(beast: Beast) {
>beast : Symbol(beast, Decl(typeGuardIntersectionTypes.ts, 64, 23))

if (beast.legs === 4) {
>beast.legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast.legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast : Symbol(beast, Decl(typeGuardIntersectionTypes.ts, 64, 23))
>legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))

log(`pegasus - 4 legs, wings`);
>log : Symbol(log, Decl(typeGuardIntersectionTypes.ts, 48, 1))
}
else if (beast.legs === 2) {
>beast.legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast.legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast : Symbol(beast, Decl(typeGuardIntersectionTypes.ts, 64, 23))
>legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))

log(`bird - 2 legs, wings`);
>log : Symbol(log, Decl(typeGuardIntersectionTypes.ts, 48, 1))
}
else {
log(`unknown - ${beast.legs} legs, wings`);
>log : Symbol(log, Decl(typeGuardIntersectionTypes.ts, 48, 1))
>beast.legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast.legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast : Symbol(beast, Decl(typeGuardIntersectionTypes.ts, 64, 23))
>legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
}
}

// All non-winged beasts with legs
else {
log(`manbearpig - ${beast.legs} legs, no wings`);
>log : Symbol(log, Decl(typeGuardIntersectionTypes.ts, 48, 1))
>beast.legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast.legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
>beast : Symbol(beast, Decl(typeGuardIntersectionTypes.ts, 64, 23))
>legs : Symbol(legs, Decl(typeGuardIntersectionTypes.ts, 55, 38), Decl(typeGuardIntersectionTypes.ts, 56, 21))
>legs : Symbol(Legged.legs, Decl(typeGuardIntersectionTypes.ts, 56, 21))
}
}

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10 changes: 5 additions & 5 deletions tests/baselines/reference/typeGuardIntersectionTypes.types
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Expand Up @@ -166,12 +166,12 @@ function identifyBeast(beast: Beast) {
if (hasWings(beast)) {
>hasWings(beast) : boolean
>hasWings : (x: Beast) => x is Winged
>beast : Beast & Legged
>beast : Legged

if (beast.legs === 4) {
>beast.legs === 4 : boolean
>beast.legs : number
>beast : Beast & Legged & Winged
>beast : Legged & Winged
>legs : number
>4 : 4

Expand All @@ -183,7 +183,7 @@ function identifyBeast(beast: Beast) {
else if (beast.legs === 2) {
>beast.legs === 2 : boolean
>beast.legs : number
>beast : Beast & Legged & Winged
>beast : Legged & Winged
>legs : number
>2 : 2

Expand All @@ -198,7 +198,7 @@ function identifyBeast(beast: Beast) {
>log : (s: string) => void
>`unknown - ${beast.legs} legs, wings` : `unknown - ${number} legs, wings`
>beast.legs : number
>beast : Beast & Legged & Winged
>beast : Legged & Winged
>legs : number
}
}
Expand All @@ -210,7 +210,7 @@ function identifyBeast(beast: Beast) {
>log : (s: string) => void
>`manbearpig - ${beast.legs} legs, no wings` : `manbearpig - ${number} legs, no wings`
>beast.legs : number
>beast : Beast & Legged
>beast : Legged
>legs : number
}
}
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12 changes: 11 additions & 1 deletion tests/baselines/reference/typeGuardsWithInstanceOf.errors.txt
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@@ -1,8 +1,12 @@
tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(7,20): error TS2339: Property 'global' does not exist on type 'never'.
The intersection 'I & RegExp' was reduced to 'never' because property 'global' has conflicting types in some constituents.
tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(31,11): error TS2339: Property 'onChanges' does not exist on type 'C | (Validator & Partial<OnChanges>)'.
Property 'onChanges' does not exist on type 'C'.
tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(32,11): error TS2339: Property 'onChanges' does not exist on type 'C | (Validator & Partial<OnChanges>)'.
Property 'onChanges' does not exist on type 'C'.


==== tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts (1 errors) ====
==== tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts (3 errors) ====
interface I { global: string; }
var result!: I;
var result2!: I;
Expand Down Expand Up @@ -37,7 +41,13 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(7,20)
}
v // Validator & Partial<OnChanges> via subtype reduction
if (v.onChanges) {
~~~~~~~~~
!!! error TS2339: Property 'onChanges' does not exist on type 'C | (Validator & Partial<OnChanges>)'.
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this looks like a regression. I reported this case back then, because the effect of instaceof affected code outside of the if statement

!!! error TS2339: Property 'onChanges' does not exist on type 'C'.
v.onChanges({});
~~~~~~~~~
!!! error TS2339: Property 'onChanges' does not exist on type 'C | (Validator & Partial<OnChanges>)'.
!!! error TS2339: Property 'onChanges' does not exist on type 'C'.
}
}

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4 changes: 0 additions & 4 deletions tests/baselines/reference/typeGuardsWithInstanceOf.symbols
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Expand Up @@ -71,14 +71,10 @@ function foo() {
>v : Symbol(v, Decl(typeGuardsWithInstanceOf.ts, 25, 7))

if (v.onChanges) {
>v.onChanges : Symbol(onChanges, Decl(typeGuardsWithInstanceOf.ts, 11, 21))
>v : Symbol(v, Decl(typeGuardsWithInstanceOf.ts, 25, 7))
>onChanges : Symbol(onChanges, Decl(typeGuardsWithInstanceOf.ts, 11, 21))

v.onChanges({});
>v.onChanges : Symbol(onChanges, Decl(typeGuardsWithInstanceOf.ts, 11, 21))
>v : Symbol(v, Decl(typeGuardsWithInstanceOf.ts, 25, 7))
>onChanges : Symbol(onChanges, Decl(typeGuardsWithInstanceOf.ts, 11, 21))
}
}

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18 changes: 9 additions & 9 deletions tests/baselines/reference/typeGuardsWithInstanceOf.types
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Expand Up @@ -65,21 +65,21 @@ function foo() {
>C : typeof C

v // Validator & Partial<OnChanges> & C
>v : Validator & Partial<OnChanges> & C
>v : C
}
v // Validator & Partial<OnChanges> via subtype reduction
>v : Validator & Partial<OnChanges>
>v : C | (Validator & Partial<OnChanges>)

if (v.onChanges) {
>v.onChanges : ((changes: Record<string, unknown>) => void) | undefined
>v : Validator & Partial<OnChanges>
>onChanges : ((changes: Record<string, unknown>) => void) | undefined
>v.onChanges : any
>v : C | (Validator & Partial<OnChanges>)
>onChanges : any

v.onChanges({});
>v.onChanges({}) : void
>v.onChanges : (changes: Record<string, unknown>) => void
>v : Validator & Partial<OnChanges>
>onChanges : (changes: Record<string, unknown>) => void
>v.onChanges({}) : any
>v.onChanges : any
>v : C | (Validator & Partial<OnChanges>)
>onChanges : any
>{} : {}
}
}
Expand Down