Fix narrowing on match with function subject

Fixes python#12998

mypy can't narrow match statements with functions subjects because the
callexpr node is not a literal node. This adds a 'dummy' literal node
that the match statement visitor can use to do the type narrowing.

The python grammar describes the the match subject as a named expression
so this uses that nameexpr node as it's literal.

mypy/checker.py

@@ -5043,8 +5043,13 @@ def visit_continue_stmt(self, s: ContinueStmt) -> None:
         return None
 
     def visit_match_stmt(self, s: MatchStmt) -> None:
+        # Create a dummy subject expression to handle cases where a match
+        # statement's subject is not a literal value which prevent us from correctly
+        # narrowing types and checking exhaustivity
+        named_subject = NameExpr("match")
         with self.binder.frame_context(can_skip=False, fall_through=0):
             subject_type = get_proper_type(self.expr_checker.accept(s.subject))
+            self.store_type(named_subject, subject_type)
 
             if isinstance(subject_type, DeletedType):
                 self.msg.deleted_as_rvalue(subject_type, s)
@@ -5061,7 +5066,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
             # The second pass narrows down the types and type checks bodies.
             for p, g, b in zip(s.patterns, s.guards, s.bodies):
                 current_subject_type = self.expr_checker.narrow_type_from_binder(
-                    s.subject, subject_type
+                    named_subject, subject_type
                 )
                 pattern_type = self.pattern_checker.accept(p, current_subject_type)
                 with self.binder.frame_context(can_skip=True, fall_through=2):
@@ -5072,7 +5077,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                         else_map: TypeMap = {}
                     else:
                         pattern_map, else_map = conditional_types_to_typemaps(
-                            s.subject, pattern_type.type, pattern_type.rest_type
+                            named_subject, pattern_type.type, pattern_type.rest_type
                         )
                         self.remove_capture_conflicts(pattern_type.captures, inferred_types)
                         self.push_type_map(pattern_map)
@@ -5100,7 +5105,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                                                 and expr.fullname == case_target.fullname
                                             ):
                                                 continue
-                                            type_map[s.subject] = type_map[expr]
+                                            type_map[named_subject] = type_map[expr]
 
                             self.push_type_map(guard_map)
                             self.accept(b)

test-data/unit/check-python310.test

@@ -1139,6 +1139,18 @@ match m:
 
 reveal_type(a)  # N: Revealed type is "builtins.str"
 
+[case testMatchCapturePatternFromFunctionReturningUnion]
+def func(arg: bool) -> str | int:
+    if arg:
+       return 1
+    return "a"
+
+match func(True):
+    case str(a):
+         reveal_type(a) # N: Revealed type is "builtins.str"
+    case a:
+         reveal_type(a) # N: Revealed type is "builtins.int"
+
 -- Guards --
 
 [case testMatchSimplePatternGuard]