`LOAD_FAST_BORROW` not being used even when safe to do so, if value is live at BB end.

This function

def f(x, y, c):
    return 1 + (x if c else y)

compiles to

  1           RESUME                   0

  2           LOAD_SMALL_INT           1
              LOAD_FAST_BORROW         2 (c)
              TO_BOOL
              POP_JUMP_IF_FALSE        9 (to L1)
              NOT_TAKEN
              LOAD_FAST_BORROW         0 (x)
              BINARY_OP                0 (+)
              RETURN_VALUE
      L1:     LOAD_FAST                1 (y)
              BINARY_OP                0 (+)
              RETURN_VALUE

Note that the load of y uses LOAD_FAST even though LOAD_FAST_BORROW is safe.

This becomes important with virtual iterators as the iterable for the loop is live at BB end.

Linked PRs

• gh-133721
• gh-148999

Start by reproducing the reported disassembly for f(x, y, c) and investigate the compiler handling of values live at a basic-block end. Compare the conditional branches and virtual-iterator case, then verify that y uses LOAD_FAST_BORROW when safe and add or update coverage for the reported behavior.