From 307a1ea89adb7436ef67f56b6fe0d4019533601c Mon Sep 17 00:00:00 2001
From: Adam Dangoor <adamdangoor@gmail.com>
Date: Thu, 17 Sep 2026 09:53:47 +0100
Subject: [PATCH] Narrow stored match patterns to concrete node types

---
 mypy/nativeparse.py   |  4 ++--
 mypy/nodes.py         |  6 +++---
 mypy/patterns.py      | 38 +++++++++++++++++++++++++-------------
 mypy/treetransform.py |  6 +++---
 4 files changed, 33 insertions(+), 21 deletions(-)

diff --git a/mypy/nativeparse.py b/mypy/nativeparse.py
index 11252e346620..2535f8b6888f 100644
--- a/mypy/nativeparse.py
+++ b/mypy/nativeparse.py
@@ -130,9 +130,9 @@
 from mypy.patterns import (
     AsPattern,
     ClassPattern,
+    ConcretePattern,
     MappingPattern,
     OrPattern,
-    Pattern,
     SequencePattern,
     SingletonPattern,
     StarredPattern,
@@ -1186,7 +1186,7 @@ def read_call_type(state: State, data: ReadBuffer) -> Type:
     return call_arg
 
 
-def read_pattern(state: State, data: ReadBuffer) -> Pattern:
+def read_pattern(state: State, data: ReadBuffer) -> ConcretePattern:
     tag = read_tag(data)
     if tag == nodes.AS_PATTERN:
         has_pattern = read_bool(data)
diff --git a/mypy/nodes.py b/mypy/nodes.py
index e64d86f0ab15..dc86550cebe4 100644
--- a/mypy/nodes.py
+++ b/mypy/nodes.py
@@ -80,7 +80,7 @@
 from mypy.visitor import ExpressionVisitor, NodeVisitor, StatementVisitor
 
 if TYPE_CHECKING:
-    from mypy.patterns import Pattern
+    from mypy.patterns import ConcretePattern
 
 
 @unique
@@ -2200,14 +2200,14 @@ class MatchStmt(Statement):
 
     subject: Expression
     subject_dummy: NameExpr | None
-    patterns: list[Pattern]
+    patterns: list[ConcretePattern]
     guards: list[Expression | None]
     bodies: list[Block]
 
     def __init__(
         self,
         subject: Expression,
-        patterns: list[Pattern],
+        patterns: list[ConcretePattern],
         guards: list[Expression | None],
         bodies: list[Block],
     ) -> None:
diff --git a/mypy/patterns.py b/mypy/patterns.py
index a01bf6acc876..2bd623579982 100644
--- a/mypy/patterns.py
+++ b/mypy/patterns.py
@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TypeVar
+from typing import TypeAlias, TypeVar
 
 from mypy_extensions import trait
 
@@ -30,10 +30,10 @@ class AsPattern(Pattern):
     # If pattern is None this is a capture pattern. If name and pattern are both none this is a
     # wildcard pattern.
     # Only name being None should not happen but also won't break anything.
-    pattern: Pattern | None
+    pattern: ConcretePattern | None
     name: NameExpr | None
 
-    def __init__(self, pattern: Pattern | None, name: NameExpr | None) -> None:
+    def __init__(self, pattern: ConcretePattern | None, name: NameExpr | None) -> None:
         super().__init__()
         self.pattern = pattern
         self.name = name
@@ -45,9 +45,9 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 class OrPattern(Pattern):
     """The pattern <pattern> | <pattern> | ..."""
 
-    patterns: list[Pattern]
+    patterns: list[ConcretePattern]
 
-    def __init__(self, patterns: list[Pattern]) -> None:
+    def __init__(self, patterns: list[ConcretePattern]) -> None:
         super().__init__()
         self.patterns = patterns
 
@@ -83,9 +83,9 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 class SequencePattern(Pattern):
     """The pattern [<pattern>, ...]"""
 
-    patterns: list[Pattern]
+    patterns: list[ConcretePattern]
 
-    def __init__(self, patterns: list[Pattern]) -> None:
+    def __init__(self, patterns: list[ConcretePattern]) -> None:
         super().__init__()
         self.patterns = patterns
 
@@ -108,11 +108,11 @@ def accept(self, visitor: PatternVisitor[T]) -> T:
 
 class MappingPattern(Pattern):
     keys: list[Expression]
-    values: list[Pattern]
+    values: list[ConcretePattern]
     rest: NameExpr | None
 
     def __init__(
-        self, keys: list[Expression], values: list[Pattern], rest: NameExpr | None
+        self, keys: list[Expression], values: list[ConcretePattern], rest: NameExpr | None
     ) -> None:
         super().__init__()
         assert len(keys) == len(values)
@@ -128,16 +128,16 @@ class ClassPattern(Pattern):
     """The pattern Cls(...)"""
 
     class_ref: RefExpr
-    positionals: list[Pattern]
+    positionals: list[ConcretePattern]
     keyword_keys: list[str]
-    keyword_values: list[Pattern]
+    keyword_values: list[ConcretePattern]
 
     def __init__(
         self,
         class_ref: RefExpr,
-        positionals: list[Pattern],
+        positionals: list[ConcretePattern],
         keyword_keys: list[str],
-        keyword_values: list[Pattern],
+        keyword_values: list[ConcretePattern],
     ) -> None:
         super().__init__()
         assert len(keyword_keys) == len(keyword_values)
@@ -148,3 +148,15 @@ def __init__(
 
     def accept(self, visitor: PatternVisitor[T]) -> T:
         return visitor.visit_class_pattern(self)
+
+
+ConcretePattern: TypeAlias = (
+    AsPattern
+    | OrPattern
+    | ValuePattern
+    | SingletonPattern
+    | SequencePattern
+    | StarredPattern
+    | MappingPattern
+    | ClassPattern
+)
diff --git a/mypy/treetransform.py b/mypy/treetransform.py
index c5e1ad44ea4c..f33cba1e3c7a 100644
--- a/mypy/treetransform.py
+++ b/mypy/treetransform.py
@@ -97,9 +97,9 @@
 from mypy.patterns import (
     AsPattern,
     ClassPattern,
+    ConcretePattern,
     MappingPattern,
     OrPattern,
-    Pattern,
     SequencePattern,
     SingletonPattern,
     StarredPattern,
@@ -735,9 +735,9 @@ def stmt(self, stmt: Statement) -> Statement:
         new.set_line(stmt)
         return new
 
-    def pattern(self, pattern: Pattern) -> Pattern:
+    def pattern(self, pattern: ConcretePattern) -> ConcretePattern:
         new = pattern.accept(self)
-        assert isinstance(new, Pattern)
+        assert isinstance(new, ConcretePattern)
         new.set_line(pattern)
         return new