From 3b284fec9e19f3c2bd9cd3f4cd190366a1718cae Mon Sep 17 00:00:00 2001
From: Martin Jesper Low Madsen <martin@martinjlowm.dk>
Date: Wed, 17 Jul 2019 23:22:41 +0200
Subject: [PATCH] Process a union or intersection type of all strings as a
 string

---
 src/LuaTransformer.ts    |  6 ++----
 src/TSHelper.ts          |  8 ++++++++
 test/unit/string.spec.ts | 16 ++++++++++++++++
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/LuaTransformer.ts b/src/LuaTransformer.ts
index 4d43d0348..4138f2af2 100644
--- a/src/LuaTransformer.ts
+++ b/src/LuaTransformer.ts
@@ -3918,10 +3918,8 @@ export class LuaTransformer {
             });
         }
 
-        switch (ownerType.flags) {
-            case ts.TypeFlags.String:
-            case ts.TypeFlags.StringLiteral:
-                return this.transformStringCallExpression(node);
+        if (tsHelper.isStringType(ownerType, this.checker, this.program)) {
+            return this.transformStringCallExpression(node);
         }
 
         // if ownerType is a array, use only supported functions
diff --git a/src/TSHelper.ts b/src/TSHelper.ts
index 0d8276aa9..13ff34e5a 100644
--- a/src/TSHelper.ts
+++ b/src/TSHelper.ts
@@ -140,6 +140,14 @@ export function isStringType(type: ts.Type, checker: ts.TypeChecker, program: ts
         }
     }
 
+    if (type.isUnion()) {
+        return type.types.every(t => isStringType(t, checker, program));
+    }
+
+    if (type.isIntersection()) {
+        return type.types.some(t => isStringType(t, checker, program));
+    }
+
     return (
         (type.flags & ts.TypeFlags.String) !== 0 ||
         (type.flags & ts.TypeFlags.StringLike) !== 0 ||
diff --git a/test/unit/string.spec.ts b/test/unit/string.spec.ts
index 3d6fc3c9f..8768818cf 100644
--- a/test/unit/string.spec.ts
+++ b/test/unit/string.spec.ts
@@ -349,6 +349,22 @@ test.each([`"foobar".length`, `"foobar".repeat(2)`, "`foo${'bar'}`.length", "`fo
     }
 );
 
+test("scoped string-union inference", () => {
+    const inp = "foo";
+
+    const result = util.transpileAndExecute(`
+        const union: string = "${inp}";
+
+        if (union === "foo" || union === "bar") {
+            return union.length;
+        }
+
+        return 0;
+    `);
+
+    expect(result).toBe(inp.length);
+});
+
 test.each([
     "function generic<T extends string>(string: T)",
     "type StringType = string; function generic<T extends StringType>(string: T)",