fix: treat ARC4 Tuples with mutable elements as mutable overall

src/puya/awst/wtypes.py

@@ -276,12 +276,18 @@ def from_types(cls, types: Iterable[WType]) -> typing.Self:
         types = tuple(types)
         if not types:
             raise ValueError("arc4.Tuple needs types")
+        immutable = True
         for typ in types:
             if not is_arc4_encoded_type(typ):
                 raise ValueError(f"Invalid type for arc4.Tuple: {typ}")
+            # this seems counterintuitive, but is necessary.
+            # despite the overall collection remaining stable, since ARC4 types
+            # are encoded as a single value, if items within the tuple can be mutated,
+            # then the overall value is also mutable
+            immutable = immutable and typ.immutable
         name = f"arc4.tuple<{','.join([t.name for t in types])}>"
         python_name = f"{constants.CLS_ARC4_TUPLE}[{', '.join(map(str, types))}]"
-        return cls(name=name, stub_name=python_name, types=types)
+        return cls(name=name, stub_name=python_name, types=types, immutable=immutable)
 
 
 @typing.final

tests/test_expected_output/arc4.test

@@ -104,3 +104,13 @@ def testC(x: C) -> None: ## E: UFixedNxM scale must be >=8 and <=64 bits, and be
 @subroutine
 def testD(x: D) -> None: ## E: BigUFixedNxM scale must be >64 and <=512 bits, and be a multiple of 8
     assert x
+
+
+## case: test_arc4_tuple_immutable
+from algopy import arc4
+
+class NotOkay(arc4.ARC4Contract):
+    @arc4.abimethod
+    def not_okay(self) -> None:
+        t = arc4.Tuple((arc4.UInt64(1), arc4.UInt8(1)))
+        t[0] = arc4.UInt64(2)  # type: ignore ## E: expression is not valid as assignment target (algopy.arc4.Tuple[algopy.arc4.UInt64, algopy.arc4.UInt8] is immutable)