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Back out from reducing template literal types with no extra texts too eagerly #58703

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21 changes: 13 additions & 8 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -18087,14 +18087,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
if (contains(types, wildcardType)) {
return wildcardType;
}
if (
texts.length === 2 && texts[0] === "" && texts[1] === ""
// literals (including string enums) are stringified below
&& !(types[0].flags & TypeFlags.Literal)
// infer T extends StringLike can't be unwrapped eagerly
&& !types[0].symbol?.declarations?.some(d => d.parent.kind === SyntaxKind.InferType)
&& isTypeAssignableTo(types[0], stringType)
) {
if (texts.length === 2 && texts[0] === "" && texts[1] === "" && (types[0].flags & TypeFlags.StringMapping)) {
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What's the reasoning for keeping the reduction around for string mapping types? Does it fix a known issue?

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no, it just keeps the behavior from #55371 as this one seems like a safe reduction and i thought that it might be worth avoiding a new type identity in a case like this

return types[0];
}
const newTypes: Type[] = [];
Expand Down Expand Up @@ -25580,6 +25573,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return false;
}

function isTemplateLiteralTypeWithSinglePlaceholderAndNoExtraTexts(type: Type): type is TemplateLiteralType {
if (!(type.flags & TypeFlags.TemplateLiteral)) {
return false;
}
const texts = (type as TemplateLiteralType).texts;
return texts.length === 2 && texts[0] === "" && texts[1] === "";
}

function isValidTypeForTemplateLiteralPlaceholder(source: Type, target: Type): boolean {
if (target.flags & TypeFlags.Intersection) {
return every((target as IntersectionType).types, t => t === emptyTypeLiteralType || isValidTypeForTemplateLiteralPlaceholder(source, t));
Expand Down Expand Up @@ -26217,6 +26218,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
}

function inferToTemplateLiteralType(source: Type, target: TemplateLiteralType) {
if (isTemplateLiteralTypeWithSinglePlaceholderAndNoExtraTexts(source) && isTemplateLiteralTypeWithSinglePlaceholderAndNoExtraTexts(target)) {
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I'm still considering if this shouldn't have some extra rules on top of this to be completely safe

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I'm not understanding why this needs to be here. After reverting #55371 the following works just fine:

declare function bar<T extends string>(x: `${T}`): T;

function baz<U extends string>(x: `${U}`) {
    return bar(x);  // Infers U

}

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Hmm, I confused myself which one of those errored after reverting #55371:

interface TypeMap {
    a: 'A'
    b: 'B'
}

declare const f: <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[T];

type F1 = <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[T];
const f1: F1 = f;  // Ok

type F2 = <T extends 'a' | 'b'>(x: `${T}`) => TypeMap[`${T}`];
const f2: F2 = f;  // Error, T is not assignable to `${T}`

Then I just knew where I could try to improve inference for this as I knew that in here `${T}` was being inferred for T based on `${T}` source and `${T}` target as that originally created an issue further down the road.

i agree, this PR is completely redundant 🤦‍♂️ this is what I get for jumping too quickly between work, this, and the kids

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Oh, I know the feeling. Your efforts are always appreciated!

inferFromTypes(source.types[0], target.types[0]);
return;
}
const matches = inferTypesFromTemplateLiteralType(source, target);
const types = target.types;
// When the target template literal contains only placeholders (meaning that inference is intended to extract
Expand Down
4 changes: 2 additions & 2 deletions tests/baselines/reference/numericStringLiteralTypes.types
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Expand Up @@ -14,8 +14,8 @@ type T2 = string & `${bigint}`; // `${bigint}
> : ^^^^^^^^^^^

type T3<T extends string> = string & `${T}`; // `${T}
>T3 : T
> : ^
>T3 : `${T}`
> : ^^^^^^

type T4<T extends string> = string & `${Capitalize<`${T}`>}`; // `${Capitalize<T>}`
>T4 : Capitalize<T>
Expand Down
8 changes: 4 additions & 4 deletions tests/baselines/reference/recursiveConditionalCrash3.types
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3 changes: 0 additions & 3 deletions tests/baselines/reference/recursiveConditionalCrash4.types
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Original file line number Diff line number Diff line change
@@ -1,8 +1,5 @@
//// [tests/cases/compiler/recursiveConditionalCrash4.ts] ////

=== Performance Stats ===
Instantiation count: 2,500

=== recursiveConditionalCrash4.ts ===
// Repros from #53783

Expand Down
20 changes: 10 additions & 10 deletions tests/baselines/reference/templateLiteralIntersection.types
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Expand Up @@ -25,8 +25,8 @@ type OriginA1 = `${A}`
> : ^^^

type OriginA2 = `${MixA}`
>OriginA2 : MixA
> : ^^^^
>OriginA2 : `${MixA}`
> : ^^^^^^^^^

type B = `${typeof a}`
>B : "a"
Expand All @@ -45,8 +45,8 @@ type OriginB1 = `${B}`
> : ^^^

type OriginB2 = `${MixB}`
>OriginB2 : MixB
> : ^^^^
>OriginB2 : `${MixB}`
> : ^^^^^^^^^

type MixC = { foo: string } & A
>MixC : MixC
Expand All @@ -55,20 +55,20 @@ type MixC = { foo: string } & A
> : ^^^^^^

type OriginC = `${MixC}`
>OriginC : MixC
> : ^^^^
>OriginC : `${MixC}`
> : ^^^^^^^^^

type MixD<T extends string> =
>MixD : T & { foo: string; }
> : ^^^^^^^^^^^ ^^^
>MixD : `${T & { foo: string; }}`
> : ^^^^^^^^^^^^^^ ^^^^^

`${T & { foo: string }}`
>foo : string
> : ^^^^^^

type OriginD = `${MixD<A & { foo: string }> & { foo: string }}`;
>OriginD : "a" & { foo: string; } & { foo: string; } & { foo: string; }
> : ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^
>OriginD : `${`${"a" & { foo: string; } & { foo: string; }}` & { foo: string; }}`
> : ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^
>foo : string
> : ^^^^^^
>foo : string
Expand Down
8 changes: 4 additions & 4 deletions tests/baselines/reference/templateLiteralIntersection3.types
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Expand Up @@ -47,8 +47,8 @@ options1[`foo/${path}`] = false;

// Lowercase<`foo/${Path}`> => `foo/${Lowercase<Path>}`
declare const lowercasePath: Lowercase<`foo/${Path}`>;
>lowercasePath : `foo/${Lowercase<Path>}`
> : ^^^^^^^^^^^^^^^^^^^^^^^^
>lowercasePath : `foo/${Lowercase<`${Path}`>}`
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

options1[lowercasePath] = false;
>options1[lowercasePath] = false : false
Expand All @@ -57,8 +57,8 @@ options1[lowercasePath] = false;
> : ^^^^^^^
>options1 : { prop: number; } & { [k: string]: boolean; }
> : ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>lowercasePath : `foo/${Lowercase<Path>}`
> : ^^^^^^^^^^^^^^^^^^^^^^^^
>lowercasePath : `foo/${Lowercase<`${Path}`>}`
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>false : false
> : ^^^^^

50 changes: 50 additions & 0 deletions tests/baselines/reference/templateLiteralTypes5.errors.txt
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@@ -0,0 +1,50 @@
templateLiteralTypes5.ts(10,7): error TS2322: Type '<T0 extends "a" | "b">(x: `${T0}`) => TypeMap[T0]' is not assignable to type 'F2'.
Type 'TypeMap[T2]' is not assignable to type 'TypeMap[`${T2}`]'.
Type 'T2' is not assignable to type '`${T2}`'.
Type '"a" | "b"' is not assignable to type '`${T2}`'.
Type '"a"' is not assignable to type '`${T2}`'.
templateLiteralTypes5.ts(13,11): error TS2322: Type 'T3' is not assignable to type '`${T3}`'.
Type '"a" | "b"' is not assignable to type '`${T3}`'.
Type '"a"' is not assignable to type '`${T3}`'.
templateLiteralTypes5.ts(14,11): error TS2322: Type '`${T3}`' is not assignable to type 'T3'.
'`${T3}`' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.
Type '"a" | "b"' is not assignable to type 'T3'.
'"a" | "b"' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.
Type '"a"' is not assignable to type 'T3'.
'"a"' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.


==== templateLiteralTypes5.ts (3 errors) ====
// https://github.com/microsoft/TypeScript/issues/55364
interface TypeMap {
a: "A";
b: "B";
}
declare const f: <T0 extends "a" | "b">(x: `${T0}`) => TypeMap[T0];
type F1 = <T1 extends "a" | "b">(x: `${T1}`) => TypeMap[T1];
const f1: F1 = f;
type F2 = <T2 extends 'a' | 'b'>(x: `${T2}`) => TypeMap[`${T2}`]
const f2: F2 = f
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before #55371 this was OK but the assignment above was not. The one above being an error was really spooky as both functions (source and the target there) were syntactically identical.

This line here doesn't come from the user report per se but rather it was created in the process of analyzing the error reported previously for that first assignment (since the error mentioned template literal type at that changed position in the indexed access).

I think it still might be possible to fix this case here, string like index types in the indexed accesses are stringified:

enum E { a = "a" }
type Foo = { [E.a]: 1 } // type Foo = { a: 1; }

So it feels like this fact could be leveraged to simplify such indexed access types like here

~~
!!! error TS2322: Type '<T0 extends "a" | "b">(x: `${T0}`) => TypeMap[T0]' is not assignable to type 'F2'.
!!! error TS2322: Type 'TypeMap[T2]' is not assignable to type 'TypeMap[`${T2}`]'.
!!! error TS2322: Type 'T2' is not assignable to type '`${T2}`'.
!!! error TS2322: Type '"a" | "b"' is not assignable to type '`${T2}`'.
!!! error TS2322: Type '"a"' is not assignable to type '`${T2}`'.

function f3<T3 extends "a" | "b">(x: T3) {
const test1: `${T3}` = x; // error, T3 could be instantiated with a string enum
~~~~~
!!! error TS2322: Type 'T3' is not assignable to type '`${T3}`'.
!!! error TS2322: Type '"a" | "b"' is not assignable to type '`${T3}`'.
!!! error TS2322: Type '"a"' is not assignable to type '`${T3}`'.
const test2: T3 = "" as `${T3}`; // error, T3 could be instantiated with a string enum
~~~~~
!!! error TS2322: Type '`${T3}`' is not assignable to type 'T3'.
!!! error TS2322: '`${T3}`' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.
!!! error TS2322: Type '"a" | "b"' is not assignable to type 'T3'.
!!! error TS2322: '"a" | "b"' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.
!!! error TS2322: Type '"a"' is not assignable to type 'T3'.
!!! error TS2322: '"a"' is assignable to the constraint of type 'T3', but 'T3' could be instantiated with a different subtype of constraint '"a" | "b"'.
}

4 changes: 2 additions & 2 deletions tests/baselines/reference/templateLiteralTypes5.symbols
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Expand Up @@ -51,12 +51,12 @@ function f3<T3 extends "a" | "b">(x: T3) {
>x : Symbol(x, Decl(templateLiteralTypes5.ts, 11, 34))
>T3 : Symbol(T3, Decl(templateLiteralTypes5.ts, 11, 12))

const test1: `${T3}` = x
const test1: `${T3}` = x; // error, T3 could be instantiated with a string enum
>test1 : Symbol(test1, Decl(templateLiteralTypes5.ts, 12, 9))
>T3 : Symbol(T3, Decl(templateLiteralTypes5.ts, 11, 12))
>x : Symbol(x, Decl(templateLiteralTypes5.ts, 11, 34))

const test2: T3 = "" as `${T3}`;
const test2: T3 = "" as `${T3}`; // error, T3 could be instantiated with a string enum
>test2 : Symbol(test2, Decl(templateLiteralTypes5.ts, 13, 9))
>T3 : Symbol(T3, Decl(templateLiteralTypes5.ts, 11, 12))
>T3 : Symbol(T3, Decl(templateLiteralTypes5.ts, 11, 12))
Expand Down
24 changes: 12 additions & 12 deletions tests/baselines/reference/templateLiteralTypes5.types
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Expand Up @@ -14,14 +14,14 @@ interface TypeMap {
declare const f: <T0 extends "a" | "b">(x: `${T0}`) => TypeMap[T0];
>f : <T0 extends "a" | "b">(x: `${T0}`) => TypeMap[T0]
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>x : T0
> : ^^
>x : `${T0}`
> : ^^^^^^^

type F1 = <T1 extends "a" | "b">(x: `${T1}`) => TypeMap[T1];
>F1 : F1
> : ^^
>x : T1
> : ^^
>x : `${T1}`
> : ^^^^^^^

const f1: F1 = f;
>f1 : F1
Expand All @@ -32,8 +32,8 @@ const f1: F1 = f;
type F2 = <T2 extends 'a' | 'b'>(x: `${T2}`) => TypeMap[`${T2}`]
>F2 : F2
> : ^^
>x : T2
> : ^^
>x : `${T2}`
> : ^^^^^^^

const f2: F2 = f
>f2 : F2
Expand All @@ -47,17 +47,17 @@ function f3<T3 extends "a" | "b">(x: T3) {
>x : T3
> : ^^

const test1: `${T3}` = x
>test1 : T3
> : ^^
const test1: `${T3}` = x; // error, T3 could be instantiated with a string enum
>test1 : `${T3}`
> : ^^^^^^^
>x : T3
> : ^^

const test2: T3 = "" as `${T3}`;
const test2: T3 = "" as `${T3}`; // error, T3 could be instantiated with a string enum
>test2 : T3
> : ^^
>"" as `${T3}` : T3
> : ^^
>"" as `${T3}` : `${T3}`
> : ^^^^^^^
>"" : ""
> : ^^
}
Expand Down
27 changes: 27 additions & 0 deletions tests/baselines/reference/templateLiteralTypes8.symbols
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@@ -0,0 +1,27 @@
//// [tests/cases/conformance/types/literal/templateLiteralTypes8.ts] ////

=== templateLiteralTypes8.ts ===
const enum E {
>E : Symbol(E, Decl(templateLiteralTypes8.ts, 0, 0))

a = "a",
>a : Symbol(E.a, Decl(templateLiteralTypes8.ts, 0, 14))

b = "b",
>b : Symbol(E.b, Decl(templateLiteralTypes8.ts, 1, 10))
}

type Stringify<T extends string> = `${T}`;
>Stringify : Symbol(Stringify, Decl(templateLiteralTypes8.ts, 3, 1))
>T : Symbol(T, Decl(templateLiteralTypes8.ts, 5, 15))
>T : Symbol(T, Decl(templateLiteralTypes8.ts, 5, 15))

let z1: `${E}` = "a";
>z1 : Symbol(z1, Decl(templateLiteralTypes8.ts, 7, 3))
>E : Symbol(E, Decl(templateLiteralTypes8.ts, 0, 0))

let z2: Stringify<E> = "a";
>z2 : Symbol(z2, Decl(templateLiteralTypes8.ts, 8, 3))
>Stringify : Symbol(Stringify, Decl(templateLiteralTypes8.ts, 3, 1))
>E : Symbol(E, Decl(templateLiteralTypes8.ts, 0, 0))

36 changes: 36 additions & 0 deletions tests/baselines/reference/templateLiteralTypes8.types
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@@ -0,0 +1,36 @@
//// [tests/cases/conformance/types/literal/templateLiteralTypes8.ts] ////

=== templateLiteralTypes8.ts ===
const enum E {
>E : E
> : ^

a = "a",
>a : E.a
> : ^^^
>"a" : "a"
> : ^^^

b = "b",
>b : E.b
> : ^^^
>"b" : "b"
> : ^^^
}

type Stringify<T extends string> = `${T}`;
>Stringify : `${T}`
> : ^^^^^^

let z1: `${E}` = "a";
>z1 : "a" | "b"
> : ^^^^^^^^^
>"a" : "a"
> : ^^^

let z2: Stringify<E> = "a";
>z2 : "a" | "b"
> : ^^^^^^^^^
>"a" : "a"
> : ^^^

4 changes: 2 additions & 2 deletions tests/cases/conformance/types/literal/templateLiteralTypes5.ts
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Expand Up @@ -14,6 +14,6 @@ type F2 = <T2 extends 'a' | 'b'>(x: `${T2}`) => TypeMap[`${T2}`]
const f2: F2 = f

function f3<T3 extends "a" | "b">(x: T3) {
const test1: `${T3}` = x
const test2: T3 = "" as `${T3}`;
const test1: `${T3}` = x; // error, T3 could be instantiated with a string enum
const test2: T3 = "" as `${T3}`; // error, T3 could be instantiated with a string enum
}
12 changes: 12 additions & 0 deletions tests/cases/conformance/types/literal/templateLiteralTypes8.ts
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@@ -0,0 +1,12 @@
// @strict: true
// @noEmit: true

const enum E {
a = "a",
b = "b",
}

type Stringify<T extends string> = `${T}`;

let z1: `${E}` = "a";
let z2: Stringify<E> = "a";