Do not report exhaustive-match after deferral (#19804)

Fixes #19791

Author: sterliakov

mypy/checker.py

@@ -5682,8 +5682,8 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                 self.push_type_map(else_map, from_assignment=False)
                 unmatched_types = else_map
 
-            if unmatched_types is not None:
-                for typ in list(unmatched_types.values()):
+            if unmatched_types is not None and not self.current_node_deferred:
+                for typ in unmatched_types.values():
                     self.msg.match_statement_inexhaustive_match(typ, s)
 
             # This is needed due to a quirk in frame_context. Without it types will stay narrowed

test-data/unit/check-python310.test

@@ -2975,3 +2975,18 @@ val: int = 8
 match val:
     case FOO:  # E: Cannot assign to final name "FOO"
         pass
+
+[case testMatchExhaustivenessWithDeferral]
+# flags: --enable-error-code exhaustive-match
+from typing import Literal
+import unknown_module  # E: Cannot find implementation or library stub for module named "unknown_module" \
+                       # N: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+
+def foo(e: Literal[0, 1]) -> None:
+    match e:
+        case 0:
+            defer
+        case 1:
+            ...
+
+defer = unknown_module.foo