TypeScriptToLua · Perryvw · Jun 11, 2023 · Jun 11, 2023 · Jun 11, 2023
diff --git a/src/transformation/utils/typescript/types.ts b/src/transformation/utils/typescript/types.ts
@@ -70,6 +70,22 @@ function isExplicitArrayType(context: TransformationContext, type: ts.Type): boo
     return false;
 }
 
+function isAlwaysExplicitArrayType(context: TransformationContext, type: ts.Type): boolean {
+    if (context.checker.isArrayType(type) || context.checker.isTupleType(type)) return true;
+    if (type.symbol) {
+        const baseConstraint = context.checker.getBaseConstraintOfType(type);
+        if (baseConstraint && baseConstraint !== type) {
+            return isAlwaysExplicitArrayType(context, baseConstraint);
+        }
+    }
+
+    if (type.isUnionOrIntersection()) {
+        return type.types.every(t => isAlwaysExplicitArrayType(context, t));
+    }
+
+    return false;
+}
+
 /**
  * Iterate over a type and its bases until the callback returns true.
  */
@@ -95,6 +111,10 @@ export function isArrayType(context: TransformationContext, type: ts.Type): bool
     return forTypeOrAnySupertype(context, type, t => isExplicitArrayType(context, t));
 }
 
+export function isAlwaysArrayType(context: TransformationContext, type: ts.Type): boolean {
+    return forTypeOrAnySupertype(context, type, t => isAlwaysExplicitArrayType(context, t));
+}
+
 export function isFunctionType(type: ts.Type): boolean {
     return type.getCallSignatures().length > 0;
 }

diff --git a/src/transformation/visitors/spread.ts b/src/transformation/visitors/spread.ts
@@ -13,7 +13,7 @@ import {
     isFunctionScopeWithDefinition,
     ScopeType,
 } from "../utils/scope";
-import { isArrayType, findFirstNonOuterParent } from "../utils/typescript";
+import { findFirstNonOuterParent, isAlwaysArrayType } from "../utils/typescript";
 import { isMultiReturnCall } from "./language-extensions/multi";
 import { isGlobalVarargConstant } from "./language-extensions/vararg";
 
@@ -96,7 +96,8 @@ export const transformSpreadElement: FunctionVisitor<ts.SpreadElement> = (node,
     }
 
     const type = context.checker.getTypeAtLocation(node.expression); // not ts-inner expression, in case of casts
-    if (isArrayType(context, type)) {
+    if (isAlwaysArrayType(context, type)) {
+        // All union members must be arrays to be able to shortcut to unpack call
         return createUnpackCall(context, innerExpression, node);
     }
 

diff --git a/test/unit/language-extensions/__snapshots__/vararg.spec.ts.snap b/test/unit/language-extensions/__snapshots__/vararg.spec.ts.snap
@@ -24,12 +24,14 @@ f(_G, ____)"
 exports[`$vararg invalid use ("function f(s: string[]) {} f($vararg);"): diagnostics 1`] = `"main.ts(2,38): error TSTL: $vararg can only be used in a spread element ('...$vararg') in global scope."`;
 
 exports[`$vararg invalid use ("function foo(...args: string[]) {} function bar() { foo(...$vararg); }"): code 1`] = `
-"function foo(self, ...)
+"local ____lualib = require("lualib_bundle")
+local __TS__Spread = ____lualib.__TS__Spread
+function foo(self, ...)
 end
 function bar(self)
     foo(
         _G,
-        table.unpack(____)
+        __TS__Spread(____)
     )
 end"
 `;

diff --git a/test/unit/spread.spec.ts b/test/unit/spread.spec.ts
@@ -561,3 +561,15 @@ test("can spread LuaSet (#1384)", () => {
     expect(result).toContain("foo");
     expect(result).toContain("bar");
 });
+
+// https://github.com/TypeScriptToLua/TypeScriptToLua/issues/1426
+test.each([true, false])("spread a decorator or array union type (#1426)", choice => {
+    util.testFunction`
+        function* things() {
+            yield "a";
+            yield "b"
+        }
+        const c: boolean = ${util.formatCode(choice)};
+        return [...(c ? things() : ["c", "d"])];
+    `.expectToMatchJsResult();
+});