Handle union of string values

Author: epabst

src/main/kotlin/converter/typeUtils.kt

@@ -115,12 +115,32 @@ private fun ObjectTypeToKotlinTypeMapper.mapUnionType(type: UnionOrIntersectionT
 
     val mappedTypes = notNullTypes.map { mapType(it, null) }
     return KtTypeUnion(when {
-        !nullable -> mappedTypes.distinct()
+        !nullable -> mappedTypes.mergeToPreventCompilationConflicts()
         notNullTypes.size == 1 -> mappedTypes.map { it.copy(isNullable = true) }
         else -> (mappedTypes + KtType(NOTHING, isNullable = true)).distinct()
     })
 }
 
+/**
+ * Normalize to a KtType such that equals will be true if the KotlinJS compiler would consider them
+ * conflicting if both were the only parameter to identically named functions.
+ */
+private fun KtType.normalizeToDetectCompilationConflicts(): KtType {
+    return copy(comment = null, typeArgs = typeArgs.map { it.normalizeToDetectCompilationConflicts() })
+}
+
+/**
+ * Handle the case where a function is overloaded with the same Kotlin parameter types by merging them.
+ * Comments are not taken into account for the comparison, but are preserved by concatenation using "|" since a "union".
+ * This is especially useful for Typescript string literal unions.
+ */
+private fun List<KtType>.mergeToPreventCompilationConflicts(): List<KtType> {
+    return groupBy { it.normalizeToDetectCompilationConflicts() }.map { entry ->
+        val comments = entry.value.mapNotNull { it.comment }
+        entry.key.copy(comment = if (comments.isEmpty()) null else comments.joinToString(" | "))
+    }
+}
+
 private inline val UnionOrIntersectionType.containsUndefined: Boolean
     get() {
         val array = types.asDynamic()

testData/unionType/stringValues.d.kt

@@ -0,0 +1,5 @@
+package stringValues
+
+external interface MapGenerator {
+    fun generate(scope: String /* "city" | "state" | "country" | "world" */)
+}

testData/unionType/stringValues.d.ts

@@ -0,0 +1,5 @@
+export type GeographicScope = 'city' | 'state' | 'country' | 'world';
+
+export interface MapGenerator {
+    generate(scope: GeographicScope)
+}