Fix incorrectly inferred Never return type for some function implementations

docs/changelog.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Fix incorrectly inferred `Never` return type for some function
+  implementations (#490)
 - Infer precise call signatures for `TypedDict` types (#487)
 - Add mechanism to prevent crashes on objects
   with unusual `__getattr__` methods (#486)

pyanalyze/implementation.py

@@ -24,6 +24,7 @@
     ParameterKind,
 )
 from .value import (
+    NO_RETURN_VALUE,
     UNINITIALIZED_VALUE,
     AnnotatedValue,
     AnySource,
@@ -98,8 +99,11 @@ def flatten_unions(
     unwrap_annotated: bool = False,
 ) -> ImplReturn:
     value_lists = [
-        flatten_values(val, unwrap_annotated=unwrap_annotated) for val in values
+        list(flatten_values(val, unwrap_annotated=unwrap_annotated)) for val in values
     ]
+    # If the lists are empty, we end up inferring Never as the return type, which
+    # generally isn't right.
+    value_lists = [lst if lst else [NO_RETURN_VALUE] for lst in value_lists]
     results = [
         clean_up_implementation_fn_return(callable(*vals))
         for vals in product(*value_lists)

pyanalyze/test_implementation.py

@@ -3,6 +3,7 @@
 from .test_node_visitor import assert_passes
 
 from .value import (
+    NO_RETURN_VALUE,
     AnnotatedValue,
     AnySource,
     AnyValue,
@@ -519,6 +520,18 @@ def capybara(x: int, y: str) -> None:
             lst += [x]
             assert_is_value(lst, GenericValue(list, [TypedValue(int)]))
 
+    @assert_passes()
+    def test_list_iadd_never(self):
+        def render_feedback_text():
+            z = []
+
+            detail_text = None
+            if detail_text:
+                assert_is_value(detail_text, NO_RETURN_VALUE)
+                z += detail_text
+
+            return z
+
     @assert_passes()
     def test_weak_value(self):
         from typing import List