Process NamedTuple decorators in semantic analyzer

mypy/semanal.py

@@ -1744,6 +1744,10 @@ def analyze_namedtuple_classdef(
                 self.setup_type_vars(defn, tvar_defs)
                 self.setup_alias_type_vars(defn)
                 with self.scope.class_scope(defn.info):
+                    for deco in defn.decorators:
+                        deco.accept(self)
+                        if isinstance(deco, RefExpr) and deco.fullname in FINAL_DECORATOR_NAMES:
+                            info.is_final = True
                     with self.named_tuple_analyzer.save_namedtuple_body(info):
                         self.analyze_class_body_common(defn)
             return True

mypyc/test-data/run-tuples.test

@@ -98,6 +98,7 @@ assert f(Sub(3, 2)) == 3
 -- Ref: https://github.com/mypyc/mypyc/issues/924
 [case testNamedTupleClassSyntax]
 from typing import Dict, List, NamedTuple, Optional, Tuple, Union
+from typing_extensions import final
 
 class FuncIR: pass
 
@@ -121,6 +122,11 @@ class Record(NamedTuple):
 # Ref: https://github.com/mypyc/mypyc/issues/938
 class ClassIR: pass
 
+# Ref: https://github.com/mypyc/mypyc/issues/927
+@final
+class Inextensible(NamedTuple):
+    x: int
+
 [file driver.py]
 from typing import ForwardRef, Optional
 from native import ClassIR, FuncIR, Record

test-data/unit/semanal-namedtuple.test

@@ -225,3 +225,23 @@ class B(A): pass
 [out]
 main:2: error: Unsupported dynamic base class "NamedTuple"
 main:2: error: Name "NamedTuple" is not defined
+
+[case testNamedTupleWithDecorator]
+from typing import final, NamedTuple
+
+@final
+class A(NamedTuple("N", [("x", int)])):
+    pass
+[builtins fixtures/tuple.pyi]
+[out]
+MypyFile:1(
+  ImportFrom:1(typing, [final, NamedTuple])
+  ClassDef:4(
+    A
+    TupleType(
+      Tuple[builtins.int, fallback=__main__.N@4])
+    Decorators(
+      NameExpr(final [typing.final]))
+    BaseType(
+      __main__.N@4)
+    PassStmt:5()))