Improve match narrowing and reachability analysis

Fixes #12534, #15878

mypy/checker.py

@@ -4944,7 +4944,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                         self.push_type_map(pattern_map)
                         self.push_type_map(pattern_type.captures)
                     if g is not None:
-                        with self.binder.frame_context(can_skip=True, fall_through=3):
+                        with self.binder.frame_context(can_skip=False, fall_through=3):
                             gt = get_proper_type(self.expr_checker.accept(g))
 
                             if isinstance(gt, DeletedType):
@@ -4953,6 +4953,21 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                             guard_map, guard_else_map = self.find_isinstance_check(g)
                             else_map = or_conditional_maps(else_map, guard_else_map)
 
+                            # If the guard narrowed the subject, copy the narrowed types over
+                            if isinstance(p, AsPattern):
+                                case_target = p.pattern or p.name
+                                if isinstance(case_target, NameExpr):
+                                    for type_map in (guard_map, else_map):
+                                        if not type_map:
+                                            continue
+                                        for expr in list(type_map):
+                                            if not (
+                                                isinstance(expr, NameExpr)
+                                                and expr.fullname == case_target.fullname
+                                            ):
+                                                continue
+                                            type_map[s.subject] = type_map[expr]
+
                             self.push_type_map(guard_map)
                             self.accept(b)
                     else:

test-data/unit/check-python310.test

@@ -1372,7 +1372,7 @@ match m:
         reveal_type(m)  # N: Revealed type is "__main__.Medal"
 
 [case testMatchNarrowUsingPatternGuardSpecialCase]
-def f(x: int | str) -> int:  # E: Missing return statement
+def f(x: int | str) -> int:
     match x:
         case x if isinstance(x, str):
             return 0
@@ -1973,3 +1973,46 @@ def f2(x: T) -> None:
         case DataFrame():  # type: ignore[misc]
             pass
 [builtins fixtures/primitives.pyi]
+
+[case testMatchGuardReachability]
+# flags: --warn-unreachable
+def f1(e: int) -> int:
+    match e:
+        case x if True:
+            return x
+        case _:
+            return 0  # E: Statement is unreachable
+    e = 0  # E: Statement is unreachable
+
+
+def f2(e: int) -> int:
+    match e:
+        case x if bool():
+            return x
+        case _:
+            return 0
+    e = 0  # E: Statement is unreachable
+
+def f3(e: int | str | bytes) -> int:
+    match e:
+        case x if isinstance(x, int):
+            return x
+        case [x]:
+            return 0  # E: Statement is unreachable
+        case str(x):
+            return 0
+    reveal_type(e)  # N: Revealed type is "builtins.bytes"
+    return 0
+
+def f4(e: int | str | bytes) -> int:
+    match e:
+        case x if isinstance(x, int):
+            pass
+        case [x]:
+            return 0  # E: Statement is unreachable
+        case str(x):
+            return 0
+    reveal_type(e)  # N: Revealed type is "Union[builtins.int, builtins.bytes]"
+    return 0
+
+[builtins fixtures/primitives.pyi]