(inappropriate) manipulation of expression objects causes segfault (PR#7326)

deepayan@stat.wisc.edu writes:

foo <- expression(alpha, beta, gamma)
foo[2]

expression(beta)

foo[2] <- NA
foo ## or str(foo)

Segmentation fault

(Same behaviour in R 1.9.1)

'foo[[2]] <- NA' works fine, though.

Right. The NA is not important; foo[2] <- 1 crashes R just as
effectively. We should likely either give an error or demote [ to [[.
The latter is what happens with lists, and expressions are basically
just list of call objects (or names or constants). To wit:

x <- list(1,2,3)
x[2] <- 1
x

[[1]]
[1] 1

[[2]]
[1] 1

[[3]]
[1] 3

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
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