[Rd] Bug on qr.coef when qr is created by a zero matrix with colnames and all y equals zero

[Kun Ren]

Dear maintainers,

I'm reporting a bug in qr.coef that mishandles the colnames of matrix. A minimal reproducible example is as follows:

x <- cbind(rep(0, 10), rep(0, 10))
y <- rep(0, 10)
q <- qr.default(x)
qr.coef(q, y)
[1] NA NA

If x has colnames, then qr.coef will end up with an error:

x <- cbind(x1 = rep(0, 10), x2 = rep(0, 10))
y <- rep(0, 10)
q <- qr.default(x)
qr.coef(q, y)
Error in qr.coef(q, y) : object 'pivotted' not found

This happens only when x any y are all zeros. I found this problem when I implement a qr-based wls which stops when all weights are zero and the x matrix has colnames.

A brief debug on qr.coef shows that when `nam` is not NULL, no branch of code leads to creating `pivotted` before using it finally. 

Best,
Kun