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Finalize evolving array types before narrowing in switch clauses #64814

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944bf5b
Finalize evolving array types before narrowing in switch clauses
Andarist Oct 14, 2026
049ff16
Make evolving array switch test cases reachable and observable
Andarist Oct 14, 2026
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6 changes: 5 additions & 1 deletion tsc/internal/checker/flow.go
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Expand Up @@ -1060,7 +1060,8 @@ func (c *Checker) getTypeAtSwitchClause(f *FlowState, flow *ast.FlowNode) FlowTy
data := flow.Node.AsFlowSwitchClauseData()
expr := ast.SkipParentheses(data.SwitchStatement.Expression())
flowType := c.getTypeAtFlowNode(f, flow.Antecedent)
t := flowType.t
nonEvolvingType := c.finalizeEvolvingArrayType(flowType.t)

@Andarist Mateusz Burzyński (Andarist) Oct 14, 2026

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this just matches the preestablished pattern used by other narrowing~ functions like getTypeAtFlowCondition, getTypeAtFlowCall and co.

t := nonEvolvingType
switch {
case c.isMatchingReference(f.reference, expr):
t = c.narrowTypeBySwitchOnDiscriminant(t, data)
Expand All @@ -1085,6 +1086,9 @@ func (c *Checker) getTypeAtSwitchClause(f *FlowState, flow *ast.FlowNode) FlowTy
t = c.narrowTypeBySwitchOnDiscriminantProperty(t, access, data)
}
}
if t == nonEvolvingType {
return flowType
}
return c.newFlowType(t, flowType.incomplete)
}

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90 changes: 90 additions & 0 deletions tsc/testdata/baselines/reference/compiler/narrowSwitchOnEvolvingArrayNoCrash1.errors.txt
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narrowSwitchOnEvolvingArrayNoCrash1.ts(2,9): error TS7034: Variable 'a' implicitly has type 'any[]' in some locations where its type cannot be determined.
narrowSwitchOnEvolvingArrayNoCrash1.ts(3,24): error TS7005: Variable 'a' implicitly has an 'any[]' type.
narrowSwitchOnEvolvingArrayNoCrash1.ts(3,34): error TS2678: Type 'number' is not comparable to type 'any[]'.
narrowSwitchOnEvolvingArrayNoCrash1.ts(7,9): error TS7034: Variable 'b' implicitly has type 'any[]' in some locations where its type cannot be determined.
narrowSwitchOnEvolvingArrayNoCrash1.ts(8,11): error TS7005: Variable 'b' implicitly has an 'any[]' type.
narrowSwitchOnEvolvingArrayNoCrash1.ts(9,10): error TS2678: Type 'number' is not comparable to type 'any[]'.
narrowSwitchOnEvolvingArrayNoCrash1.ts(15,9): error TS7034: Variable 'b' implicitly has type 'any[]' in some locations where its type cannot be determined.
narrowSwitchOnEvolvingArrayNoCrash1.ts(16,7): error TS7005: Variable 'b' implicitly has an 'any[]' type.
narrowSwitchOnEvolvingArrayNoCrash1.ts(16,7): error TS2367: This comparison appears to be unintentional because the types 'any[]' and 'number' have no overlap.
narrowSwitchOnEvolvingArrayNoCrash1.ts(22,7): error TS7034: Variable 'c' implicitly has type 'any[]' in some locations where its type cannot be determined.
narrowSwitchOnEvolvingArrayNoCrash1.ts(23,18): error TS7005: Variable 'c' implicitly has an 'any[]' type.
narrowSwitchOnEvolvingArrayNoCrash1.ts(26,13): error TS2322: Type 'string[]' is not assignable to type 'number[]'.
Type 'string' is not assignable to type 'number'.
narrowSwitchOnEvolvingArrayNoCrash1.ts(32,7): error TS7034: Variable 'c' implicitly has type 'any[]' in some locations where its type cannot be determined.
narrowSwitchOnEvolvingArrayNoCrash1.ts(33,14): error TS7005: Variable 'c' implicitly has an 'any[]' type.
narrowSwitchOnEvolvingArrayNoCrash1.ts(35,11): error TS2322: Type 'string[]' is not assignable to type 'number[]'.
Type 'string' is not assignable to type 'number'.


==== narrowSwitchOnEvolvingArrayNoCrash1.ts (15 errors) ====
function f1() {
const a = [];
~
!!! error TS7034: Variable 'a' implicitly has type 'any[]' in some locations where its type cannot be determined.
while (true) switch (a) { case 0: }
~
!!! error TS7005: Variable 'a' implicitly has an 'any[]' type.
~
!!! error TS2678: Type 'number' is not comparable to type 'any[]'.
}

function f2() {
const b = [];
~
!!! error TS7034: Variable 'b' implicitly has type 'any[]' in some locations where its type cannot be determined.
switch (b) {
~
!!! error TS7005: Variable 'b' implicitly has an 'any[]' type.
case 0:
~
!!! error TS2678: Type 'number' is not comparable to type 'any[]'.
b;
}
}

function f3() {
const b = [];
~
!!! error TS7034: Variable 'b' implicitly has type 'any[]' in some locations where its type cannot be determined.
if (b === 0) {
~
!!! error TS7005: Variable 'b' implicitly has an 'any[]' type.
~~~~~~~
!!! error TS2367: This comparison appears to be unintentional because the types 'any[]' and 'number' have no overlap.
b;
}
}

function f4() {
let c = [];
~
!!! error TS7034: Variable 'c' implicitly has type 'any[]' in some locations where its type cannot be determined.
switch (typeof c) {
~
!!! error TS7005: Variable 'c' implicitly has an 'any[]' type.
case "object": {
c.push("x");
const arr: number[] = c;
~~~
!!! error TS2322: Type 'string[]' is not assignable to type 'number[]'.
!!! error TS2322: Type 'string' is not assignable to type 'number'.
}
}
}

function f5() {
let c = [];
~
!!! error TS7034: Variable 'c' implicitly has type 'any[]' in some locations where its type cannot be determined.
if (typeof c === "object") {
~
!!! error TS7005: Variable 'c' implicitly has an 'any[]' type.
c.push("x");
const arr: number[] = c;
~~~
!!! error TS2322: Type 'string[]' is not assignable to type 'number[]'.
!!! error TS2322: Type 'string' is not assignable to type 'number'.
}
}

84 changes: 84 additions & 0 deletions tsc/testdata/baselines/reference/compiler/narrowSwitchOnEvolvingArrayNoCrash1.symbols
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//// [tests/cases/compiler/narrowSwitchOnEvolvingArrayNoCrash1.ts] ////

=== narrowSwitchOnEvolvingArrayNoCrash1.ts ===
function f1() {
>f1 : Symbol(f1, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 0, 0))

const a = [];
>a : Symbol(a, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 1, 7))

while (true) switch (a) { case 0: }
>a : Symbol(a, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 1, 7))
}

function f2() {
>f2 : Symbol(f2, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 3, 1))

const b = [];
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 6, 7))

switch (b) {
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 6, 7))

case 0:
b;
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 6, 7))
}
}

function f3() {
>f3 : Symbol(f3, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 11, 1))

const b = [];
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 14, 7))

if (b === 0) {
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 14, 7))

b;
>b : Symbol(b, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 14, 7))
}
}

function f4() {
>f4 : Symbol(f4, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 18, 1))

let c = [];
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 21, 5))

switch (typeof c) {
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 21, 5))

case "object": {
c.push("x");
>c.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 21, 5))
>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))

const arr: number[] = c;
>arr : Symbol(arr, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 25, 11))
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 21, 5))
}
}
}

function f5() {
>f5 : Symbol(f5, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 28, 1))

let c = [];
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 31, 5))

if (typeof c === "object") {
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 31, 5))

c.push("x");
>c.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 31, 5))
>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))

const arr: number[] = c;
>arr : Symbol(arr, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 34, 9))
>c : Symbol(c, Decl(narrowSwitchOnEvolvingArrayNoCrash1.ts, 31, 5))
}
}

105 changes: 105 additions & 0 deletions tsc/testdata/baselines/reference/compiler/narrowSwitchOnEvolvingArrayNoCrash1.types
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//// [tests/cases/compiler/narrowSwitchOnEvolvingArrayNoCrash1.ts] ////

=== narrowSwitchOnEvolvingArrayNoCrash1.ts ===
function f1() {
>f1 : () => void

const a = [];
>a : any[]
>[] : never[]

while (true) switch (a) { case 0: }
>true : true
>a : any[]
>0 : 0
}

function f2() {
>f2 : () => void

const b = [];
>b : any[]
>[] : never[]

switch (b) {
>b : any[]

case 0:
>0 : 0

b;
>b : never
}
}

function f3() {
>f3 : () => void

const b = [];
>b : any[]
>[] : never[]

if (b === 0) {
>b === 0 : boolean
>b : any[]
>0 : 0

b;
>b : never
}
}

function f4() {
>f4 : () => void

let c = [];
>c : any[]
>[] : never[]

switch (typeof c) {
>typeof c : "bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"
>c : any[]

case "object": {
>"object" : "object"

c.push("x");
>c.push("x") : number
>c.push : (...items: any[]) => number
>c : any[]
>push : (...items: any[]) => number
>"x" : "x"

const arr: number[] = c;
>arr : number[]
>c : string[]
}
}
}

function f5() {
>f5 : () => void

let c = [];
>c : any[]
>[] : never[]

if (typeof c === "object") {
>typeof c === "object" : boolean
>typeof c : "bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"
>c : any[]
>"object" : "object"

c.push("x");
>c.push("x") : number
>c.push : (...items: any[]) => number
>c : any[]
>push : (...items: any[]) => number
>"x" : "x"

const arr: number[] = c;
>arr : number[]
>c : string[]
}
}

41 changes: 41 additions & 0 deletions tsc/testdata/tests/cases/compiler/narrowSwitchOnEvolvingArrayNoCrash1.ts
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// @target: es2015
// @strict: true
// @noEmit: true

function f1() {
const a = [];
while (true) switch (a) { case 0: }
}

function f2() {
const b = [];
switch (b) {
case 0:
b;
}
}

function f3() {
const b = [];
if (b === 0) {
b;
}
}

function f4() {
let c = [];
switch (typeof c) {
case "object": {
c.push("x");
const arr: number[] = c;
}
}
}

function f5() {
let c = [];
if (typeof c === "object") {
c.push("x");
const arr: number[] = c;
}
}