GeezerVison

…osed one

select._match_expr rewrote A.select(A.T == x) as select.valueeq(A, x):
its clause "args[0] is parent or args[0].T is parent" matched the
transpose of the parent, and the rewrite then compared A itself, dropping
the transpose and, for a matrix that is not square, the dimension check:

    S = Matrix.from_coo([0, 1], [1, 0], [1, 5], nrows=2, ncols=2)
    S.select(S.T == 1)
    before: (0, 1) = 1
    after:  (1, 0) = 5, as S.select((S.T == 1).new()) gives
    B (2 x 3): B.select(B.T == 1)
    before: a 2 x 3 result
    after:  DimensionMismatch

The clause is now "args[0] is parent"; anything else takes the mask path.
On main since #252, for built-in dtypes. "Select a UDT's elements equal or
unequal to a value, as == compares them" sent UDT eq and ne through the
same clause, which had made A.select(A.T == x) on a UDT a wrong answer
where it raised before. test_udt_select_by_equality now uses a 2 x 3
matrix with entries off the diagonal, and a mask with replace.