Only instantiate types if we need to

ahejlsberg · ahejlsberg · commit 31551fd0aecd · 2019-04-29T17:17:16.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -10383,11 +10383,11 @@ namespace ts {
             // We attempt to resolve the conditional type only when the check and extends types are non-generic
             if (!checkTypeInstantiable && !maybeTypeOfKind(inferredExtendsType, TypeFlags.Instantiable | TypeFlags.GenericMappedType)) {
                 if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown) {
-                    return instantiateType(root.trueType, combinedMapper || mapper);
+                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
                 }
                 // Return union of trueType and falseType for 'any' since it matches anything
                 if (checkType.flags & TypeFlags.Any) {
-                    return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), falseType]);
+                    return getUnionType([combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType, falseType]);
                 }
                 // Return falseType for a definitely false extends check. We check an instantiations of the two
                 // types with type parameters mapped to the wildcard type, the most permissive instantiations
@@ -10402,7 +10402,7 @@ namespace ts {
                 //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
                 // doesn't immediately resolve to 'string' instead of being deferred.
                 if (isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {
-                    return instantiateType(root.trueType, combinedMapper || mapper);
+                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
                 }
             }
             // Return a deferred type for a check that is neither definitely true nor definitely false

Original file line number	Diff line number	Diff line change
`@@ -10383,11 +10383,11 @@ namespace ts {`
`10383`	`10383`	`// We attempt to resolve the conditional type only when the check and extends types are non-generic`
`10384`	`10384`	`if (!checkTypeInstantiable && !maybeTypeOfKind(inferredExtendsType, TypeFlags.Instantiable \| TypeFlags.GenericMappedType)) {`
`10385`	`10385`	`if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown) {`
`10386`		`- return instantiateType(root.trueType, combinedMapper \|\| mapper);`
	`10386`	`+ return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;`
`10387`	`10387`	`}`
`10388`	`10388`	`// Return union of trueType and falseType for 'any' since it matches anything`
`10389`	`10389`	`if (checkType.flags & TypeFlags.Any) {`
`10390`		`- return getUnionType([instantiateType(root.trueType, combinedMapper \|\| mapper), falseType]);`
	`10390`	`+ return getUnionType([combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType, falseType]);`
`10391`	`10391`	`}`
`10392`	`10392`	`// Return falseType for a definitely false extends check. We check an instantiations of the two`
`10393`	`10393`	`// types with type parameters mapped to the wildcard type, the most permissive instantiations`
`@@ -10402,7 +10402,7 @@ namespace ts {`
`10402`	`10402`	`// type Foo<T extends { x: any }> = T extends { x: string } ? string : number`
`10403`	`10403`	`// doesn't immediately resolve to 'string' instead of being deferred.`
`10404`	`10404`	`if (isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {`
`10405`		`- return instantiateType(root.trueType, combinedMapper \|\| mapper);`
	`10405`	`+ return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;`
`10406`	`10406`	`}`
`10407`	`10407`	`}`
`10408`	`10408`	`// Return a deferred type for a check that is neither definitely true nor definitely false`