Assigning unpredefined SYMTAB index

[Miguel Pineiro Jr.]

Hello, everyone.

Fedora 36
Kernel 5.17.13-300.fc36.x86_64 
GNU Awk 5.1.1, API: 3.1 (GNU MPFR 4.1.0-p13, GNU MP 6.2.1)

The gawk manual states [1] that as of gawk 5.0 an attempt to assign to 
SYMTAB[index] when index isn't already defined produces a fatal error. This 
isn't completely accurate. It can be done for an array.

As documented, assignment to SYMTAB["a"] fails fatally when a is undefined, but 
assignment to SYMTAB["a"]["b"] succeeds and creates the array SYMTAB["a"] (but 
without creating a global variable).

gawk 'BEGIN {
    if (!("a" in SYMTAB))
        print "a is not predefined";
    SYMTAB["a"]["b"] = 123;
    if ("a" in SYMTAB)
        print "SYMTAB[\"a\"] array exists, length:", length(SYMTAB["a"]);
    if ("b" in SYMTAB["a"])
        print "SYMTAB[\"a\"][\"b\"] exists: " SYMTAB["a"]["b"];
}'

Output:
a is not predefined
SYMTAB["a"] array exists, length: 1
SYMTAB["a"]["b"] exists: 123


The array can even be named SYMTAB, but SYMTAB["SYMTAB"] isn't 
self-referential. This array's membership seems to have nothing to do with the 
global variable namespace.

gawk 'BEGIN {
    a=1;
    SYMTAB["SYMTAB"]["a"] = 2;
    print a;
    print SYMTAB["a"];
    print SYMTAB["SYMTAB"]["a"];
}'

Output:
1
1
2

This hasn't caused me any problems and none of my "real" scripts are affected. 
I just came across this while working on nawk bugs and comparing behavior 
across implementations.

If this isn't a bug, please excuse the distraction.

Take care,
Miguel

[1] 
https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html#index-differences-in-awk-and-gawk-39