sys.monitoring branches for `match` cases showing incorrectly

Bug report

Bug description:

With the code from PR 122564, the destinations of branches into the blocks of certain cases are mapping to the pattern as well, making them look like they go out of scope:

v = 1
match v:
    case 1:
        x = 1
    case 2:
        x = 2
    case 3:
        x = 3
x += 2

This code yields (branches listing below extracted from co_branches()):

  0          0       RESUME                   0

  1          2       LOAD_CONST               0 (1)
             4       STORE_NAME               0 (v)

  2          6       LOAD_NAME                0 (v)

  3          8       COPY                     1
            10       LOAD_CONST               0 (1)
            12       COMPARE_OP              88 (bool(==))
            16       POP_JUMP_IF_FALSE        5 (to L1)
            20       NOT_TAKEN
            22       POP_TOP

  4         24       LOAD_CONST               0 (1)
            26       STORE_NAME               1 (x)
            28       JUMP_FORWARD            19 (to L3)

  5   L1:   30       COPY                     1
            32       LOAD_CONST               1 (2)
            34       COMPARE_OP              88 (bool(==))
            38       POP_JUMP_IF_FALSE        5 (to L2)
            42       NOT_TAKEN
            44       POP_TOP

  6         46       LOAD_CONST               1 (2)
            48       STORE_NAME               1 (x)
            50       JUMP_FORWARD             8 (to L3)

  7   L2:   52       LOAD_CONST               2 (3)
            54       COMPARE_OP              88 (bool(==))
            58       POP_JUMP_IF_FALSE        3 (to L3)
            62       NOT_TAKEN

  8         64       LOAD_CONST               2 (3)
            66       STORE_NAME               1 (x)

  9   L3:   68       LOAD_NAME                1 (x)
            70       LOAD_CONST               1 (2)
            72       BINARY_OP               13 (+=)
            76       STORE_NAME               1 (x)
            78       RETURN_CONST             3 (None)
'<module>' branches:
    ex8.py 3:9-3:10 "1" (<module>@16) -> 3:9-3:10 "1" (<module>@22) [case -> out]
    ex8.py 3:9-3:10 "1" (<module>@16) -> 5:9-5:10 "2" (<module>@30) [case -> next case]
    ex8.py 5:9-5:10 "2" (<module>@38) -> 5:9-5:10 "2" (<module>@44) [case -> out]
    ex8.py 5:9-5:10 "2" (<module>@38) -> 7:9-7:10 "3" (<module>@52) [case -> next case]
    ex8.py 7:9-7:10 "3" (<module>@58) -> 8:12-8:13 "3" (<module>@64) [case -> body]
    ex8.py 7:9-7:10 "3" (<module>@58) -> 9:0-9:1 "x" (<module>@68) [case -> next]

@markshannon @nedbat

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-130627

match-case の例を使って分岐一覧を再現し、sys.monitoring の co_branches() エントリーポイントと、PR 122564 で導入された動作を調べます。case パターンと本体について報告される遷移先を比較します。完了の条件は、case ブロックへの分岐がパターンの位置に対応付けられなくなり、誤ってスコープを抜けるように見えなくなることです。