Detect comparisons between large unions or intersections

src/compiler/checker.ts

@@ -16892,7 +16892,6 @@ namespace ts {
                     return isIdenticalTo(source, target);
                 }
 
-
                 // We fastpath comparing a type parameter to exactly its constraint, as this is _super_ common,
                 // and otherwise, for type parameters in large unions, causes us to need to compare the union to itself,
                 // as we break down the _target_ union first, _then_ get the source constraint - so for every
@@ -16953,6 +16952,8 @@ namespace ts {
                     return Ternary.False;
                 }
 
+                traceUnionsOrIntersectionsTooLarge(source, target);
+
                 let result = Ternary.False;
                 const saveErrorInfo = captureErrorCalculationState();
 
@@ -17088,11 +17089,41 @@ namespace ts {
                 }
             }
 
+            function traceUnionsOrIntersectionsTooLarge(source: Type, target: Type): void {
+                if (!tracing.isTracing()) {
+                    return;
+                }
+
+                if ((source.flags & TypeFlags.UnionOrIntersection) && (target.flags & TypeFlags.UnionOrIntersection)) {
+                    const sourceUnionOrIntersection = source as UnionOrIntersectionType;
+                    const targetUnionOrIntersection = target as UnionOrIntersectionType;
+
+                    if (sourceUnionOrIntersection.objectFlags & targetUnionOrIntersection.objectFlags & ObjectFlags.PrimitiveUnion) {
+                        // There's a fast path for comparing primitive unions
+                        return;
+                    }
+
+                    const sourceSize = sourceUnionOrIntersection.types.length;
+                    const targetSize = targetUnionOrIntersection.types.length;
+                    if (sourceSize * targetSize > 1E6) {
+                        tracing.instant(tracing.Phase.CheckTypes, "traceUnionsOrIntersectionsTooLarge_DepthLimit", {
+                            sourceId: source.id,
+                            sourceSize,
+                            targetId: target.id,
+                            targetSize,
+                            pos: errorNode?.pos,
+                            end: errorNode?.end
+                        });
+                    }
+                }
+            }
+
             function isIdenticalTo(source: Type, target: Type): Ternary {
                 const flags = source.flags & target.flags;
                 if (!(flags & TypeFlags.Substructure)) {
                     return Ternary.False;
                 }
+                traceUnionsOrIntersectionsTooLarge(source, target);
                 if (flags & TypeFlags.UnionOrIntersection) {
                     let result = eachTypeRelatedToSomeType(<UnionOrIntersectionType>source, <UnionOrIntersectionType>target);
                     if (result) {