Fix list concatenation not using outer list type context

Author: Ria-K912

mypy/checkexpr.py

@@ -3608,6 +3608,12 @@ def visit_op_expr(self, e: OpExpr) -> Type:
         if e.op == "*" and isinstance(e.left, ListExpr):
             # Expressions of form [...] * e get special type inference.
             return self.check_list_multiply(e)
+        if e.op == "+":
+            # Expressions of form list + list under a list type context get special
+            # type inference so literals/comprehensions honor the outer context.
+            ctx = get_proper_type(self.type_context[-1])
+            if is_named_instance(ctx, "builtins.list"):
+                return self.check_list_add(e)
         if e.op == "%":
             if isinstance(e.left, BytesExpr):
                 return self.strfrm_checker.check_str_interpolation(e.left, e.right)
@@ -4505,6 +4511,23 @@ def check_list_multiply(self, e: OpExpr) -> Type:
         e.method_type = method_type
         return result
 
+    def check_list_add(self, e: OpExpr) -> Type:
+        """Type check list concatenation under an outer list type context.
+
+        Like list literals and '[...] * n', both operands should see the outer
+        list[...] context so that e.g. ``x: list[int | None] = [0] + [1]`` works.
+        """
+        ctx = self.type_context[-1]
+        left_type = self.accept(e.left, type_context=ctx)
+        right_type = self.accept(e.right, type_context=ctx)
+        # check_op re-accepts the right operand; keep the context-aware type.
+        with self.type_overrides_set([e.right], [right_type]):
+            result, method_type = self.check_op(
+                "__add__", left_type, e.right, e, allow_reverse=True
+            )
+        e.method_type = method_type
+        return result
+
     def visit_assignment_expr(self, e: AssignmentExpr) -> Type:
         value = self.accept(e.value)
         binder_version = self.chk.binder.version

test-data/unit/check-inference-context.test

@@ -849,6 +849,18 @@ if int():
     a = [''] * 3  # E: List item 0 has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
 
+[case testListAddInContext]
+from typing import List, Optional
+x: List[Optional[int]]
+x = [0, 1, 2] + [10]
+x = [i for i in [0, 1, 2]] + [10]
+x = [0] + [None]
+x = [] + [1]
+x = [1] + [2] + [3]
+y: List[int]
+y = [0] + ['x']  # E: List item 0 has incompatible type "str"; expected "int"
+[builtins fixtures/list.pyi]
+
 [case testUnionTypeContext]
 from typing import Union, List, TypeVar
 T = TypeVar('T')