From 79a73bf51029a8b59bc792063827689e05192e55 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 25 Oct 2021 15:05:06 -0700
Subject: [PATCH] Always cache relations involving intersection types

---
 src/compiler/checker.ts | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 217eb47cfa..ed75651e6e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -18207,13 +18207,12 @@ namespace ts {
                 let result = Ternary.False;
                 const saveErrorInfo = captureErrorCalculationState();
 
-                // Note that these checks are specifically ordered to produce correct results. In particular,
-                // we need to deconstruct unions before intersections (because unions are always at the top),
-                // and we need to handle "each" relations before "some" relations for the same kind of type.
-                if (source.flags & TypeFlags.UnionOrIntersection || target.flags & TypeFlags.UnionOrIntersection) {
-                    result = getConstituentCount(source) * getConstituentCount(target) >= 4 ?
-                        recursiveTypeRelatedTo(source, target, reportErrors, intersectionState | IntersectionState.UnionIntersectionCheck, recursionFlags) :
-                        structuredTypeRelatedTo(source, target, reportErrors, intersectionState | IntersectionState.UnionIntersectionCheck);
+                if ((source.flags & TypeFlags.Union || target.flags & TypeFlags.Union) && getConstituentCount(source) * getConstituentCount(target) < 4) {
+                    // We skip caching when source or target is a union with no more than three constituents.
+                    result = structuredTypeRelatedTo(source, target, reportErrors, intersectionState | IntersectionState.UnionIntersectionCheck);
+                }
+                else if (source.flags & TypeFlags.UnionOrIntersection || target.flags & TypeFlags.UnionOrIntersection) {
+                    result = recursiveTypeRelatedTo(source, target, reportErrors, intersectionState | IntersectionState.UnionIntersectionCheck, recursionFlags);
                 }
                 if (!result && !(source.flags & TypeFlags.Union) && (source.flags & (TypeFlags.StructuredOrInstantiable) || target.flags & TypeFlags.StructuredOrInstantiable)) {
                     if (result = recursiveTypeRelatedTo(source, target, reportErrors, intersectionState, recursionFlags)) {
@@ -18677,17 +18676,17 @@ namespace ts {
                 const maybeStart = maybeCount;
                 maybeKeys[maybeCount] = id;
                 maybeCount++;
+                const saveExpandingFlags = expandingFlags;
                 if (recursionFlags & RecursionFlags.Source) {
                     sourceStack[sourceDepth] = source;
                     sourceDepth++;
+                    if (!(expandingFlags & ExpandingFlags.Source) && isDeeplyNestedType(source, sourceStack, sourceDepth)) expandingFlags |= ExpandingFlags.Source;
                 }
                 if (recursionFlags & RecursionFlags.Target) {
                     targetStack[targetDepth] = target;
                     targetDepth++;
+                    if (!(expandingFlags & ExpandingFlags.Target) && isDeeplyNestedType(target, targetStack, targetDepth)) expandingFlags |= ExpandingFlags.Target;
                 }
-                const saveExpandingFlags = expandingFlags;
-                if (!(expandingFlags & ExpandingFlags.Source) && isDeeplyNestedType(source, sourceStack, sourceDepth)) expandingFlags |= ExpandingFlags.Source;
-                if (!(expandingFlags & ExpandingFlags.Target) && isDeeplyNestedType(target, targetStack, targetDepth)) expandingFlags |= ExpandingFlags.Target;
                 let originalHandler: typeof outofbandVarianceMarkerHandler;
                 let propagatingVarianceFlags: RelationComparisonResult = 0;
                 if (outofbandVarianceMarkerHandler) {
@@ -18713,13 +18712,13 @@ namespace ts {
                 if (outofbandVarianceMarkerHandler) {
                     outofbandVarianceMarkerHandler = originalHandler;
                 }
-                expandingFlags = saveExpandingFlags;
                 if (recursionFlags & RecursionFlags.Source) {
                     sourceDepth--;
                 }
                 if (recursionFlags & RecursionFlags.Target) {
                     targetDepth--;
                 }
+                expandingFlags = saveExpandingFlags;
                 if (result) {
                     if (result === Ternary.True || (sourceDepth === 0 && targetDepth === 0)) {
                         if (result === Ternary.True || result === Ternary.Maybe) {
@@ -23150,7 +23149,7 @@ namespace ts {
         }
 
         function getConstituentCount(type: Type) {
-            return type.flags & TypeFlags.UnionOrIntersection ? (type as UnionOrIntersectionType).types.length : 1;
+            return type.flags & TypeFlags.Union ? (type as UnionType).types.length : 1;
         }
 
         function extractTypesOfKind(type: Type, kind: TypeFlags) {