fix: passing a mutable value more than once to a subroutine will now …

…raise an error as allowing it would break semantic compatability

BREAKING CHANGE:
passing a mutable value more than once to a subroutine causes an error

src/puya/ir/builder/callsub.py

@@ -2,6 +2,7 @@
 
 import attrs
 
+from puya import log
 from puya.awst import (
     nodes as awst_nodes,
     wtypes,
@@ -13,6 +14,8 @@
 from puya.ir.models import InvokeSubroutine, Register, Subroutine, Value, ValueProvider, ValueTuple
 from puya.parse import SourceLocation
 
+logger = log.get_logger(__name__)
+
 
 def visit_subroutine_call_expression(
     context: IRFunctionBuildContext, expr: awst_nodes.SubroutineCallExpression
@@ -46,6 +49,11 @@ def _call_subroutine(
         arg_val = arg_lookup.get(index=idx, param_name=param.name)
         resolved_args.append(arg_val)
         if param.implicit_return:
+            if arg_val in implicit_args:
+                logger.error(
+                    "mutable values cannot be passed more than once to a subroutine",
+                    location=arg_val.source_location,
+                )
             implicit_args.append(arg_val)
     if not arg_lookup.is_empty:
         raise CodeError("function call arguments do not match signature", call_location) from None

tests/test_expected_output/arc4.test

@@ -589,3 +589,21 @@ class MyTest(ARC4Contract):
         this_is_ok = some_address[0]
         assert this_is_ok == 0
         some_address[0] = arc4.Byte(123) ## E: expression is not valid as an assignment target - object is immutable
+
+## case: test_mutable_params_passed_only_once
+from algopy import *
+
+class MyTest(ARC4Contract):
+    @arc4.abimethod
+    def test(self) -> None:
+        my_array = arc4.DynamicBytes()
+        self.double_use_not_allowed(my_array, my_array) ## E: mutable values cannot be passed more than once to a subroutine
+        # if we allowed this, the following would fail
+        assert my_array[0] == 42
+        assert my_array[1] == 43
+
+
+    @subroutine
+    def double_use_not_allowed(self, arr1: arc4.DynamicBytes, arr2: arc4.DynamicBytes) -> None:
+        arr1.append(arc4.Byte(42))
+        arr2.append(arc4.Byte(43))