diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts index b57a505c169..d64d1e5da3b 100644 --- a/src/compiler/checker.ts +++ b/src/compiler/checker.ts @@ -20320,14 +20320,17 @@ namespace ts { } function getGlobalNonNullableTypeInstantiation(type: Type) { + // First reduce away any constituents that are assignable to 'undefined' or 'null'. This not only eliminates + // 'undefined' and 'null', but also higher-order types such as a type parameter 'U extends undefined | null' + // that isn't eliminated by a NonNullable instantiation. + const reducedType = getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull); if (!deferredGlobalNonNullableTypeAlias) { deferredGlobalNonNullableTypeAlias = getGlobalSymbol("NonNullable" as __String, SymbolFlags.TypeAlias, /*diagnostic*/ undefined) || unknownSymbol; } - // Use NonNullable global type alias if available to improve quick info/declaration emit - if (deferredGlobalNonNullableTypeAlias !== unknownSymbol) { - return getTypeAliasInstantiation(deferredGlobalNonNullableTypeAlias, [type]); - } - return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull); // Type alias unavailable, fall back to non-higher-order behavior + // If the NonNullable type is available, return an instantiation. Otherwise just return the reduced type. + return deferredGlobalNonNullableTypeAlias !== unknownSymbol ? + getTypeAliasInstantiation(deferredGlobalNonNullableTypeAlias, [reducedType]) : + reducedType; } function getNonNullableType(type: Type): Type {