fix: Use froundeven for lowering float roundings

guppylang/src/guppylang/std/num.py

@@ -448,8 +448,13 @@ def __rmod__(self: float, other: float) -> float: ...
     @custom_function(checker=ReversingChecker(), unitary_flags=UnitaryFlags.Dagger)
     def __rmul__(self: float, other: float) -> float: ...
 
-    @hugr_op(float_op("fround"), unitary_flags=UnitaryFlags.Dagger)  # TODO
-    def __round__(self: float) -> float: ...
+    @hugr_op(float_op("froundeven"), unitary_flags=UnitaryFlags.Dagger)  # TODO
+    def ___round__hugr(self: float) -> float: ...
+
+    @guppy
+    @no_type_check
+    def __round__(self: float) -> int:
+        return int(self.___round__hugr())
 
     @custom_function(checker=ReversingChecker(), unitary_flags=UnitaryFlags.Dagger)
     def __rpow__(self: float, other: float) -> float: ...

tests/integration/test_arithmetic.py

@@ -405,3 +405,29 @@ def rem() -> int:
 
     run_int_fn(quot, -1)
     run_int_fn(rem, 3)
+
+
+def test_round(run_int_fn, run_float_fn_approx) -> None:
+    """Asserts that the `round` prelude function behaves like a drop-in replacement for
+    the Python built-in variant."""
+
+    @guppy
+    def int_round() -> int:
+        return round(1)  # noqa: RUF057
+
+    @guppy
+    def nat_round() -> int:
+        return round(nat(2))
+
+    @guppy
+    def float_round_tie_down() -> int:
+        return round(2.5)
+
+    @guppy
+    def float_round_tie_up() -> int:
+        return round(3.5)
+
+    run_int_fn(int_round, 1)
+    run_int_fn(nat_round, 2)
+    run_int_fn(float_round_tie_down, 2)
+    run_int_fn(float_round_tie_up, 4)