fix Math.atan2 emitting math.atan2 instead of math.atan for Lua 5.4+ (#1700)

RealColdFry · web-flow · commit 84ef0b2d09b8 · 2026-04-01T20:50:15.000+02:00
* fix Math.atan2 emitting math.atan2 instead of math.atan for Lua 5.4+

math.atan2 was removed in Lua 5.3 (replaced by two-arg math.atan).
The codegen only special-cased 5.3 but not 5.4 or 5.5, which also
lack math.atan2 (5.4 has it behind a compat flag, 5.5 drops it).

* invert atan2 condition to default future Lua targets to math.atan

Enumerate the older targets that need math.atan2 instead of the newer
ones that need math.atan, so any future Lua target (5.6+) defaults to
the modern behavior rather than silently regressing.
diff --git a/src/transformation/builtins/math.ts b/src/transformation/builtins/math.ts
@@ -42,14 +42,20 @@ export function transformMathCall(
 
     const expressionName = calledMethod.name.text;
     switch (expressionName) {
-        // Lua 5.3: math.atan(y, x)
+        // Lua 5.3+: math.atan(y, x)
         // Otherwise: math.atan2(y, x)
         case "atan2": {
             if (context.luaTarget === LuaTarget.Universal) {
                 return transformLuaLibFunction(context, LuaLibFeature.MathAtan2, node, ...params);
             }
 
-            const method = lua.createStringLiteral(context.luaTarget === LuaTarget.Lua53 ? "atan" : "atan2");
+            const useAtan2 =
+                context.luaTarget === LuaTarget.Lua50 ||
+                context.luaTarget === LuaTarget.Lua51 ||
+                context.luaTarget === LuaTarget.Lua52 ||
+                context.luaTarget === LuaTarget.LuaJIT ||
+                context.luaTarget === LuaTarget.Luau;
+            const method = lua.createStringLiteral(useAtan2 ? "atan2" : "atan");
             return lua.createCallExpression(lua.createTableIndexExpression(math, method), params, node);
         }
 
diff --git a/test/unit/builtins/math.spec.ts b/test/unit/builtins/math.spec.ts
@@ -95,8 +95,8 @@ util.testEachVersion("Math.atan2", () => util.testExpression`Math.atan2(4, 5)`,
     [tstl.LuaTarget.Lua51]: builder => builder.tap(expectMathAtan2),
     [tstl.LuaTarget.Lua52]: builder => builder.tap(expectMathAtan2),
     [tstl.LuaTarget.Lua53]: builder => builder.tap(expectMathAtan),
-    [tstl.LuaTarget.Lua54]: builder => builder.tap(expectMathAtan2),
-    [tstl.LuaTarget.Lua55]: builder => builder.tap(expectMathAtan2),
+    [tstl.LuaTarget.Lua54]: builder => builder.tap(expectMathAtan),
+    [tstl.LuaTarget.Lua55]: builder => builder.tap(expectMathAtan),
     [tstl.LuaTarget.Luau]: builder => builder.tap(expectMathAtan2),
 });
 
