Comment 12 for bug 1950283

How to check if R has been built without Fortran complex functions:

$ sudo apt install --no-install-recommends r-base

$ R -e "eigen(print(cbind(c(0, 1i), c(-1i, 0))))"

Expected output:

> eigen(print(cbind(c(0, 1i), c(-1i, 0))))
     [,1] [,2]
[1,] 0+0i 0-1i
[2,] 0+1i 0+0i
eigen() decomposition
$values
[1] 1 -1

$vectors
                      [,1] [,2]
[1,] -0.7071068+0.0000000i -0.7071068+0.0000000i
[2,] 0.0000000-0.7071068i 0.0000000+0.7071068i

Output without Fortran complex functions:

> eigen(print(cbind(c(0, 1i), c(-1i, 0))))
     [,1] [,2]
[1,] 0+0i 0-1i
[2,] 0+1i 0+0i
Error in eigen(print(cbind(c(0, 0+1i), c(-(0+1i), 0)))) :
  Fortran complex functions are not available on this platform
Execution halted

Optionally, follow the instructions to download upstream's R packages from CRAN:
https://cloud.r-project.org/bin/linux/ubuntu/