Fix narrowing on match with function subject (python#16503)

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.

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

mypy/checker.py

@@ -5053,6 +5053,19 @@ def visit_continue_stmt(self, s: ContinueStmt) -> None:
         return
 
     def visit_match_stmt(self, s: MatchStmt) -> None:
+        named_subject: Expression
+        if isinstance(s.subject, CallExpr):
+            # Create a dummy subject expression to handle cases where a match statement's subject
+            # is not a literal value. This lets us correctly narrow types and check exhaustivity
+            # This is hack!
+            id = s.subject.callee.fullname if isinstance(s.subject.callee, RefExpr) else ""
+            name = "dummy-match-" + id
+            v = Var(name)
+            named_subject = NameExpr(name)
+            named_subject.node = v
+        else:
+            named_subject = s.subject
+
         with self.binder.frame_context(can_skip=False, fall_through=0):
             subject_type = get_proper_type(self.expr_checker.accept(s.subject))
 
@@ -5071,7 +5084,7 @@ 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):
@@ -5082,7 +5095,7 @@ 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)
@@ -5110,7 +5123,7 @@ 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 +1139,21 @@ match m:
 
 reveal_type(a)  # N: Revealed type is "builtins.str"
 
+[case testMatchCapturePatternFromFunctionReturningUnion]
+def func1(arg: bool) -> str | int: ...
+def func2(arg: bool) -> bytes | int: ...
+
+def main() -> None:
+    match func1(True):
+        case str(a):
+            match func2(True):
+                case c:
+                    reveal_type(a)  # N: Revealed type is "builtins.str"
+                    reveal_type(c)  # N: Revealed type is "Union[builtins.bytes, builtins.int]"
+            reveal_type(a)  # N: Revealed type is "builtins.str"
+        case a:
+            reveal_type(a)  # N: Revealed type is "builtins.int"
+
 -- Guards --
 
 [case testMatchSimplePatternGuard]