From 1541653e55b4b7e7367dd14bd963f12895aa491a Mon Sep 17 00:00:00 2001
From: malkarjun <145407983+malkarjun@users.noreply.github.com>
Date: Thu, 2 Oct 2025 17:39:48 -0400
Subject: [PATCH 1/2] errorcodes: added optional code missing-return-annotation
 and enforce in semantic analysis file; added tests as well

---
 mypy/errorcodes.py                            |  7 +++++++
 mypy/semanal.py                               | 21 ++++++++++++++++++-
 .../unit/check-missing-return-annotation.test |  8 +++++++
 3 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 test-data/unit/check-missing-return-annotation.test

diff --git a/mypy/errorcodes.py b/mypy/errorcodes.py
index a96f5f723a7d2..810ae7b3f351d 100644
--- a/mypy/errorcodes.py
+++ b/mypy/errorcodes.py
@@ -319,5 +319,12 @@ def __hash__(self) -> int:
     default_enabled=False,
 )
 
+MISSING_RETURN_ANNOTATION = ErrorCode(
+    "missing-return-annotation", 
+    "Function has no return type annotation",
+    "Optional",
+)
+
+
 # This copy will not include any error codes defined later in the plugins.
 mypy_error_codes = error_codes.copy()
diff --git a/mypy/semanal.py b/mypy/semanal.py
index 17dc9bfadc1f2..37913a2967652 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -57,7 +57,7 @@
 
 from mypy import errorcodes as codes, message_registry
 from mypy.constant_fold import constant_fold_expr
-from mypy.errorcodes import PROPERTY_DECORATOR, ErrorCode
+from mypy.errorcodes import PROPERTY_DECORATOR, ErrorCode, MISSING_RETURN_ANNOTATION
 from mypy.errors import Errors, report_internal_error
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.message_registry import ErrorMessage
@@ -926,6 +926,25 @@ def visit_func_def(self, defn: FuncDef) -> None:
             with self.inside_except_star_block_set(value=False):
                 self.analyze_func_def(defn)
 
+                # Missing return annotation check (optional error code)
+                from mypy.types import CallableType
+
+                if (
+                    # Makes sure return type is not specified, ignores lambdas and special cases
+                    isinstance(defn.type, CallableType)
+                    and defn.type.ret_type is None
+                    and any(arg.type_annotation is not None for arg in defn.arguments)
+                    and defn.name != "__init__"
+                    and not defn.is_lambda
+                    and not defn.is_overload
+                ):
+                    self.msg.fail(
+                        f'Function "{defn.name}" has no return type annotation',
+                        defn,
+                        code=MISSING_RETURN_ANNOTATION,
+                    )
+        
+
     def function_fullname(self, fullname: str) -> str:
         if self.current_overload_item is None:
             return fullname
diff --git a/test-data/unit/check-missing-return-annotation.test b/test-data/unit/check-missing-return-annotation.test
new file mode 100644
index 0000000000000..1c4d74fe60132
--- /dev/null
+++ b/test-data/unit/check-missing-return-annotation.test
@@ -0,0 +1,8 @@
+[case test_missing_return_annotation_ok]
+# flags: --enable-error-code missing-return-annotation
+def g(x: int) -> int:
+    return x
+
+[case test_missing_return_annotation_off_by_default]
+def h(x: int):
+    return x  # no error expected, since code is opt-in

From 671e083205e764c2453f601e51c43250910dcb37 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 2 Oct 2025 21:56:59 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/errorcodes.py | 4 +---
 mypy/semanal.py    | 3 +--
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/mypy/errorcodes.py b/mypy/errorcodes.py
index 810ae7b3f351d..87c6057449602 100644
--- a/mypy/errorcodes.py
+++ b/mypy/errorcodes.py
@@ -320,9 +320,7 @@ def __hash__(self) -> int:
 )
 
 MISSING_RETURN_ANNOTATION = ErrorCode(
-    "missing-return-annotation", 
-    "Function has no return type annotation",
-    "Optional",
+    "missing-return-annotation", "Function has no return type annotation", "Optional"
 )
 
 
diff --git a/mypy/semanal.py b/mypy/semanal.py
index 37913a2967652..792ab35c79bd6 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -57,7 +57,7 @@
 
 from mypy import errorcodes as codes, message_registry
 from mypy.constant_fold import constant_fold_expr
-from mypy.errorcodes import PROPERTY_DECORATOR, ErrorCode, MISSING_RETURN_ANNOTATION
+from mypy.errorcodes import MISSING_RETURN_ANNOTATION, PROPERTY_DECORATOR, ErrorCode
 from mypy.errors import Errors, report_internal_error
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.message_registry import ErrorMessage
@@ -943,7 +943,6 @@ def visit_func_def(self, defn: FuncDef) -> None:
                         defn,
                         code=MISSING_RETURN_ANNOTATION,
                     )
-        
 
     def function_fullname(self, fullname: str) -> str:
         if self.current_overload_item is None: