add a new forward-compatibility test

Author: MichaelChirico

inst/tests/tests.Rraw

@@ -2292,24 +2292,26 @@ RHS = as.integer(DT$a)
 test(754.06, DT[,a:=RHS,verbose=TRUE], output="RHS for item 1 has been duplicated")
 # Expand ALTREPS in assign.c, #5400
 # String conversion gets deferred
-test(754.07, DT[,a:=as.character(a),verbose=TRUE], output="RHS for item 1 has been duplicated")
+## first, a regression test of R itself -- we want to make sure our own test continues to be useful & testing its intended purpose
+test(754.07, { a = 1:10; .Internal(inspect(a)); b = as.character(a); .Internal(inspect(b))}, output = "\\bcompact\\b.*\\bdeferred string conversion\\b")
+test(754.08, DT[,a:=as.character(a),verbose=TRUE], output="RHS for item 1 has been duplicated")
 # Executing the code inside of test expands the ALTREP so we repeat the code
 # in order to check the result after a further assignment
 DT = data.table(a=1:3)
 DT[,b:=as.character(a)]
 DT[, a:=5L]
-test(754.08, DT, data.table(a=5L, b=as.character(1:3)))
+test(754.09, DT, data.table(a=5L, b=as.character(1:3)))
 # This function returns an ALTREP wrapper if the input is at least length 64
 testFun = function(x) {
   x[FALSE] = 1
   x
 }
 DT = data.table(id=1:64, col1=0, col2=0)
-test(754.09, DT[, col1 := testFun(col2), verbose = TRUE], output="RHS for item 1 has been duplicated")
+test(754.10, DT[, col1 := testFun(col2), verbose = TRUE], output="RHS for item 1 has been duplicated")
 DT = data.table(id=1:64, col1=0, col2=0)
 DT[, col1 := testFun(col2)]
 DT[, col2 := 999]
-test(754.10, DT, data.table(id=1:64, col1=0, col2=999))
+test(754.11, DT, data.table(id=1:64, col1=0, col2=999))
 rm(testFun)
 
 # Used to test warning on redundant by (#2282) but by=.EACHI has now superseded