Fix not None return types, fixes #124

Author: davidhalter

crates/zuban_python/src/type_/operations.rs

@@ -673,6 +673,18 @@ impl Type {
                 execute_type_of_type(i_s, args, result_context, on_type_error, cls.as_ref())
             }
             Type::Union(union) => Inferred::gather_simplified_union(i_s, |gather| {
+                let result_context = if result_context.expect_not_none()
+                    && union.iter().any(|t| match t {
+                        // If there's any callable with a return type that is not None we should
+                        // not complain with the lint that a None return does not make sense,
+                        // because it can be some other type.
+                        Type::Callable(c) => c.return_type != Type::None,
+                        _ => false,
+                    }) {
+                    &mut ResultContext::ExpectUnused
+                } else {
+                    result_context
+                };
                 for entry in union.iter() {
                     gather(entry.execute(i_s, None, args, result_context, on_type_error))
                 }

crates/zuban_python/tests/mypylike/tests/functions.test

@@ -433,3 +433,30 @@ def generator():
 
 val = foo()  # E: "foo" does not return a value (it only ever returns None)
 assert_type(val, None)
+
+[case function_does_not_return_value_mypy_compatible]
+# flags: --mypy-compatible
+from collections.abc import Callable
+
+def get(fn: Callable[[], None] | Callable[[], str]) -> None:
+    ret = fn()
+    reveal_type(ret)  # N: Revealed type is "None | str"
+
+def get2(fn: Callable[[float], None] | Callable[[int], None]) -> None:
+    ret = fn(1)  # E: Function does not return a value (it only ever returns None) \
+                 # E: Function does not return a value (it only ever returns None)
+    reveal_type(ret)  # N: Revealed type is "Any"
+
+[case function_does_not_return_value_no_mypy_compatible]
+# flags: --no-mypy-compatible
+# From GH #124
+from collections.abc import Callable
+
+def get(fn: Callable[[], None] | Callable[[], str]) -> None:
+    ret = fn()
+    reveal_type(ret)  # N: Revealed type is "None | str"
+
+def get2(fn: Callable[[float], None] | Callable[[int], None]) -> None:
+    ret = fn(1)  # E: Function does not return a value (it only ever returns None) \
+                 # E: Function does not return a value (it only ever returns None)
+    reveal_type(ret)  # N: Revealed type is "None"