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fix(eslint-plugin): [prefer-nullish-coalescing] handle generic type #11019

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fix(eslint-plugin): [prefer-nullish-coalescing] handle generic type #11019

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48 changes: 34 additions & 14 deletions packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ import {
isNodeOfTypes,
isNullLiteral,
isNullableType,
isTypeFlagSet,
isUndefinedIdentifier,
nullThrows,
NullThrowsReasons,
Expand Down Expand Up @@ -211,7 +212,10 @@ export default createRule<Options, MessageIds>({
* a nullishness check, taking into account the rule's configuration.
*/
function isTypeEligibleForPreferNullish(type: ts.Type): boolean {
if (!isNullableType(type)) {
if (
!isNullableType(type) &&
!isTypeFlagSet(type, ts.TypeFlags.TypeParameter)
) {
return false;
}

Expand All @@ -235,16 +239,7 @@ export default createRule<Options, MessageIds>({
return true;
}

// if the type is `any` or `unknown` we can't make any assumptions
// about the value, so it could be any primitive, even though the flags
// won't be set.
//
// technically, this is true of `void` as well, however, it's a TS error
// to test `void` for truthiness, so we don't need to bother checking for
// it in valid code.
if (
tsutils.isTypeFlagSet(type, ts.TypeFlags.Any | ts.TypeFlags.Unknown)
) {
if (isIndeterminateType(type)) {
return false;
}

Expand All @@ -254,7 +249,7 @@ export default createRule<Options, MessageIds>({
.some(t =>
tsutils
.intersectionTypeParts(t)
.some(t => tsutils.isTypeFlagSet(t, ignorableFlags)),
.some(t => isTypeFlagSet(t, ignorableFlags)),
)
) {
return false;
Expand Down Expand Up @@ -447,12 +442,12 @@ export default createRule<Options, MessageIds>({
const type = parserServices.getTypeAtLocation(
nullishCoalescingLeftNode,
);
const flags = getTypeFlags(type);

if (flags & (ts.TypeFlags.Any | ts.TypeFlags.Unknown)) {
if (isIndeterminateType(type)) {
return false;
}

const flags = getTypeFlags(type);
const hasNullType = (flags & ts.TypeFlags.Null) !== 0;

// it is fixable if we check for undefined and the type is not nullable
Expand Down Expand Up @@ -932,3 +927,28 @@ function formatComments(
)
.join('');
}

/**
* Returns `true` if the provided type is `any`, `unknown`,
* or a union type that includes a type parameter.
*
* In these cases, no assumptions can be made about the value's structure or behavior,
* as it might represent `undefined`, `null`, or any primitive,
* even if the standard type flags are not set.
*
* Note: This could technically include `void` as well, but testing for `void` is unnecessary,
* since evaluating `void` for truthiness results in a TypeScript error.
*/
function isIndeterminateType(type: ts.Type): boolean {
if (isTypeFlagSet(type, ts.TypeFlags.Any | ts.TypeFlags.Unknown)) {
return true;
}

return tsutils
.typeParts(type)
.some(t =>
tsutils
.unionTypeParts(t)
.some(t => isTypeFlagSet(t, ts.TypeFlags.TypeParameter)),
);
}
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