sq <- function(y){ y**2 }

call <- match.call(expand.dots = FALSE)
call[[1]] <- as.name('sq')
call$y <- 0.2
call <- call[c(1,3)]
eval(call)

call <- match.call(expand.dots = FALSE)
call[[1]] <- as.name('sq')
call$y <- as.name('sin')
call$x <- 0.2
call <- call[c(1,3:4)]
eval(call)

Error in sq(y = sin, x = 0.2) : unused argument (x = 0.2)