fix: Merge compatible definitions in union types

Most validation keywords apply to only one of the basic types, so a
"string" type and an "array" type can share a definition without
colliding as a ["string", "array"] type, as long as they don't have any
incompatibilities with each other. Modify UnionTypeFormatter to collapse
these disjoint types into a single definition, without using anyOf.

src/TypeFormatter/UnionTypeFormatter.ts

@@ -4,6 +4,7 @@ import { SubTypeFormatter } from "../SubTypeFormatter";
 import { BaseType } from "../Type/BaseType";
 import { UnionType } from "../Type/UnionType";
 import { TypeFormatter } from "../TypeFormatter";
+import { mergeDefinitions } from "../Utils/mergeDefinitions";
 import { uniqueArray } from "../Utils/uniqueArray";
 
 export class UnionTypeFormatter implements SubTypeFormatter {
@@ -45,6 +46,18 @@ export class UnionTypeFormatter implements SubTypeFormatter {
             }
         }
 
+        for (let idx = 0; idx < flattenedDefinitions.length - 1; idx++) {
+            for (let comp = idx + 1; comp < flattenedDefinitions.length; ) {
+                const merged = mergeDefinitions(flattenedDefinitions[idx], flattenedDefinitions[comp]);
+                if (merged) {
+                    flattenedDefinitions[idx] = merged;
+                    flattenedDefinitions.splice(comp, 1);
+                } else {
+                    comp++;
+                }
+            }
+        }
+
         return flattenedDefinitions.length > 1
             ? {
                   anyOf: flattenedDefinitions,

src/Utils/mergeDefinitions.ts

@@ -0,0 +1,25 @@
+import { Definition } from "../Schema/Definition";
+import { uniqueArray } from "./uniqueArray";
+
+/**
+ * Attempt to merge two disjoint definitions into one. Definitions are disjoint
+ * (and therefore mergeable) if all of the following are true:
+ * 1) Each has a 'type' property, and they share no types in common,
+ * 2) The cross-type validation properties 'enum' and 'const' are not on either definition, and
+ * 3) The two definitions have no properties besides 'type' in common.
+ *
+ * Returns the merged definition, or null if the two defs were not disjoint.
+ */
+export function mergeDefinitions(def1: Definition, def2: Definition): Definition | null {
+    const { type: type1, ...props1 } = def1;
+    const { type: type2, ...props2 } = def2;
+    const types = [type1!, type2!].flat();
+    if (!type1 || !type2 || uniqueArray(types).length !== types.length) {
+        return null;
+    }
+    const keys = [Object.keys(props1), Object.keys(props2)].flat();
+    if (keys.includes("enum") || keys.includes("const") || uniqueArray(keys).length !== keys.length) {
+        return null;
+    }
+    return { type: types, ...props1, ...props2 };
+}