Limit size of union types resulting from intersection type normalization

Author: ahejlsberg

src/compiler/checker.ts

@@ -9908,6 +9908,12 @@ namespace ts {
                     else {
                         // We are attempting to construct a type of the form X & (A | B) & Y. Transform this into a type of
                         // the form X & A & Y | X & B & Y and recursively reduce until no union type constituents remain.
+                        // If the estimated size of the resulting union type exceeds 100000 constituents, report an error.
+                        const size = reduceLeft(typeSet, (n, t) => n * (t.flags & TypeFlags.Union ? (<UnionType>t).types.length : 1), 1);
+                        if (size >= 100000) {
+                            error(currentNode, Diagnostics.Expression_produces_a_union_type_that_is_too_complex_to_represent);
+                            return errorType;
+                        }
                         const unionIndex = findIndex(typeSet, t => (t.flags & TypeFlags.Union) !== 0);
                         const unionType = <UnionType>typeSet[unionIndex];
                         result = getUnionType(map(unionType.types, t => getIntersectionType(replaceElement(typeSet, unionIndex, t))),