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Fix switch-true case-true narrowing to behave equivalently to default case #59459

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Fix switch-true case-true narrowing to behave equivalently to default case #59459

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5731bdc
Fix switch-true case-true narrowing to behave equivalently to default…
Zzzen Jul 29, 2024
d0ed82b
Merge branch 'main' into fix-switch-true
RyanCavanaugh Jun 26, 2025
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3 changes: 2 additions & 1 deletion src/compiler/binder.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1718,7 +1718,8 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
preSwitchCaseFlow = currentFlow;
bind(node.caseBlock);
addAntecedent(postSwitchLabel, currentFlow);
const hasDefault = forEach(node.caseBlock.clauses, c => c.kind === SyntaxKind.DefaultClause);
const hasDefault = forEach(node.caseBlock.clauses, c => c.kind === SyntaxKind.DefaultClause) ||
(skipParentheses(node.expression).kind === SyntaxKind.TrueKeyword && node.caseBlock.clauses.some(clause => clause.kind === SyntaxKind.CaseClause && skipParentheses(clause.expression).kind === SyntaxKind.TrueKeyword));
// We mark a switch statement as possibly exhaustive if it has no default clause and if all
// case clauses have unreachable end points (e.g. they all return). Note, we no longer need
// this property in control flow analysis, it's there only for backwards compatibility.
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46 changes: 46 additions & 0 deletions tests/baselines/reference/narrowByClauseExpressionInSwitchTrue11.symbols
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@@ -0,0 +1,46 @@
//// [tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts] ////

=== narrowByClauseExpressionInSwitchTrue11.ts ===
function uhoh(input: string) : "A"|"B"|"C" {
>uhoh : Symbol(uhoh, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 0))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))

switch (true) {
case /a/.test(input):
>/a/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))

return "A";
case /b/.test(input):
>/b/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))

return "B";
case true:
return "C";
}
}

function uhoh1(input: string) : "A"|"B"|"C" {
>uhoh1 : Symbol(uhoh1, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 9, 1))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))

switch ((true)) {
case /a/.test(input):
>/a/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))

return "A";
case /b/.test(input):
>/b/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))

return "B";
case (true):
return "C";
}
}
110 changes: 110 additions & 0 deletions tests/baselines/reference/narrowByClauseExpressionInSwitchTrue11.types
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@@ -0,0 +1,110 @@
//// [tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts] ////

=== narrowByClauseExpressionInSwitchTrue11.ts ===
function uhoh(input: string) : "A"|"B"|"C" {
>uhoh : (input: string) => "A" | "B" | "C"
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

switch (true) {
>true : true
> : ^^^^

case /a/.test(input):
>/a/.test(input) : boolean
> : ^^^^^^^
>/a/.test : (string: string) => boolean
> : ^ ^^ ^^^^^
>/a/ : RegExp
> : ^^^^^^
>test : (string: string) => boolean
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

return "A";
>"A" : "A"
> : ^^^

case /b/.test(input):
>/b/.test(input) : boolean
> : ^^^^^^^
>/b/.test : (string: string) => boolean
> : ^ ^^ ^^^^^
>/b/ : RegExp
> : ^^^^^^
>test : (string: string) => boolean
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

return "B";
>"B" : "B"
> : ^^^

case true:
>true : true
> : ^^^^

return "C";
>"C" : "C"
> : ^^^
}
}

function uhoh1(input: string) : "A"|"B"|"C" {
>uhoh1 : (input: string) => "A" | "B" | "C"
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

switch ((true)) {
>(true) : true
> : ^^^^
>true : true
> : ^^^^

case /a/.test(input):
>/a/.test(input) : boolean
> : ^^^^^^^
>/a/.test : (string: string) => boolean
> : ^ ^^ ^^^^^
>/a/ : RegExp
> : ^^^^^^
>test : (string: string) => boolean
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

return "A";
>"A" : "A"
> : ^^^

case /b/.test(input):
>/b/.test(input) : boolean
> : ^^^^^^^
>/b/.test : (string: string) => boolean
> : ^ ^^ ^^^^^
>/b/ : RegExp
> : ^^^^^^
>test : (string: string) => boolean
> : ^ ^^ ^^^^^
>input : string
> : ^^^^^^

return "B";
>"B" : "B"
> : ^^^

case (true):
>(true) : true
> : ^^^^
>true : true
> : ^^^^

return "C";
>"C" : "C"
> : ^^^
}
}
24 changes: 24 additions & 0 deletions tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts
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@@ -0,0 +1,24 @@
// @strict: true
// @noEmit: true

function uhoh(input: string) : "A"|"B"|"C" {
switch (true) {
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@jakebailey jakebailey Jun 30, 2025

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Awful, but, what about switch (false) + case false:?

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@Zzzen Zzzen Jul 18, 2025 •
edited
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To my surprise, it seems some folks are writing switch(false). No case false: found though. Guess it wouldn't hurt to support it too! https://github.com/search?q=switch%28false%29%7B+language%3Atypescript&type=code&p=1

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@Zzzen Zzzen Jul 18, 2025

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However, after some research, I don’t think we want to support it since switch(false) isn’t something we generally support.

playground

type A = { type: "A" };
type B = { type: "B" };
type AorB = A | B;

const isA = (x: AorB): x is A => x.type === "A";
const isB = (x: AorB): x is B => x.type === "B";

function test1(x: AorB) {
  switch (false) {
    case isA(x):
      // in a perfect world, x would have type B
      x;
      break;
    case isB(x):
      // in a perfect world, x would have type A
      x;
      break;
  }
}

function test2(x: AorB) {
  switch (true) {
    case isA(x):
      x;
      break;
    case isB(x):
      x;
      break;
  }
}

case /a/.test(input):
return "A";
case /b/.test(input):
return "B";
case true:
return "C";
}
}

function uhoh1(input: string) : "A"|"B"|"C" {
switch ((true)) {
case /a/.test(input):
return "A";
case /b/.test(input):
return "B";
case (true):
return "C";
}
}