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feat: downlevel type in index signatures #84

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feat: downlevel type in index signatures #84

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19 changes: 19 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -294,6 +294,25 @@ type T = [/** foo */ number, /** bar */ string];
The downlevel semantics are exactly the same as the original, but
the TypeScript language service won't be able to show the member names.

### `{ [key: T]: A }` (4.4)

Typescript 4.4 supports template literals as index signature types:

```ts
type O = { [key: `ak${string}`]: A };
```

becomes

```ts
type O = { [key: string]: A };
```

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Please add a Semantics section. Specifically, the transformation as written makes sense for cases where

  1. the type is a template literal
  2. the type is reference to a template literal
    because x${number}y -> string (for example) is more permissive.

I'm not sure it makes sense for the following types that also work in 4.4:

  1. symbol
  2. a reference to symbol
  3. unions of string, symbol, number or template literals.
  4. a reference to unions of string, symbol, number or template literals.

cases (2), (4) and (6) are syntactically ambiguous, so I don't think they can be downlevelled with this tool.
(3) can't be downlevelled at all.
(5) can be downlevelled to multiple index signatures if the symbol isn't in the union.

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I've updated it to satisfy only the first case.

Regarding making it compatible with the second case, can we add a flag for "unsafe" transformations or allow transformations provided by user?

#### Semantics

The downlevel d.ts will be less strict because an index signature would
have a more general type (`string`) instead of the original template literal type.

### `in out T` (4.7)

Typescript 4.7 supports variance annotations on type parameter declarations:
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3 changes: 3 additions & 0 deletions baselines/reference/ts3.4/test.d.ts
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Expand Up @@ -71,3 +71,6 @@ export declare const foo: {
export type IR = IteratorResult<number>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts3.5/test.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -71,3 +71,6 @@ export declare const foo: {
export type IR = IteratorResult<number>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts3.6/test.d.ts
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Expand Up @@ -73,3 +73,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts3.7/test.d.ts
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Expand Up @@ -73,3 +73,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts3.8/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts3.9/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.0/test.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = string;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.1/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.2/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.3/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: string]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.4/test.d.ts
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Expand Up @@ -74,3 +74,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: `${string}abc${string}`]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.5/test.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -71,3 +71,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: `${string}abc${string}`]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.6/test.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -71,3 +71,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: `${string}abc${string}`]: number;
};
3 changes: 3 additions & 0 deletions baselines/reference/ts4.7/test.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -71,3 +71,6 @@ export declare const foo: {
export type IR = IteratorResult<number, string>;
/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;
export type TTemplateLiteralIndexSignature = {
[key: `${string}abc${string}`]: number;
};
20 changes: 20 additions & 0 deletions index.js
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Expand Up @@ -367,6 +367,26 @@ function doTransform(checker, targetVersion, k) {
ts.unescapeLeadingUnderscores(member.name.escapedText),
/*hasTrailingNewline*/ false
);
} else if (
semver.lt(targetVersion, "4.4.0") &&
ts.isIndexSignatureDeclaration(n) &&
ts.SyntaxKind.TemplateLiteralType === n.parameters[0].type.kind
) {
const [p] = n.parameters;
return ts.factory.createIndexSignature(
n.modifiers,
[
ts.factory.createParameterDeclaration(
p.modifiers,
p.dotDotDotToken,
p.name,
p.questionToken,
ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
p.initializer
)
],
n.type
);
} else if (semver.lt(targetVersion, "4.7.0") && ts.isTypeParameterDeclaration(n)) {
return ts.factory.createTypeParameterDeclaration(
n.modifiers?.filter(
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2 changes: 2 additions & 0 deletions test/test.d.ts
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Expand Up @@ -78,3 +78,5 @@ export type IR = IteratorResult<number, string>;

/** Template Literal - supported since 4.1 < should be StringKeyword */
export type TTemplateLiteral = `${string}abc${string}`;

export type TTemplateLiteralIndexSignature = { [key: `${string}abc${string}`]: number }