fix: Ensure ints can be treated as booleans (#709)

Closes #681

guppylang/checker/expr_checker.py

@@ -1069,9 +1069,7 @@ def to_bool(node: ast.expr, node_ty: Type, ctx: Context) -> tuple[ast.expr, Type
         return node, node_ty
     synth = ExprSynthesizer(ctx)
     exp_sig = FunctionType([FuncInput(node_ty, InputFlags.Inout)], bool_type())
-    return synth.synthesize_instance_func(
-        node, [node], "__bool__", "truthy", exp_sig, True
-    )
+    return synth.synthesize_instance_func(node, [], "__bool__", "truthy", exp_sig, True)
 
 
 def synthesize_comprehension(

tests/integration/test_if.py

@@ -274,3 +274,27 @@ def foo() -> bool:
         return x == y
 
     validate(foo)
+
+
+def test_zero_as_bool(validate):
+
+    @compile_guppy
+    def foo() -> bool:
+        if 0:
+            return False
+        else:
+            return True
+
+    validate(foo)
+
+
+def test_one_as_bool(validate):
+
+    @compile_guppy
+    def foo() -> bool:
+        if 1:
+            return True
+        else:
+            return False
+
+    validate(foo)