gawk 5.1.1 internal error from line 1349 in eval.c

[Ed Morton]

Running on cygwin with:

   $ $SHELL --version
   GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)

   $ awk --version
   GNU Awk 5.1.1, API: 3.1 (GNU MPFR 4.1.0, GNU MP 6.2.1)

I was reviewing someone else's code that has a user-provided comparison 
function to sort alphanumeric strings like "foo123" alphabetically by 
the alphabetic part and numerically by the numeric part so that "foo6" 
would come before "foo10". I wasn't sure if the results of the match() 
in their first function were strings or strnums so I added a loop in the 
first function to dump each value in a[] and it's type after the match()::

   $ cat tst.awk
   function str_num_to_a(i, a) {
        match(i, /"([^0-9]*)([0-9]+)"/, a)
        for (x in a) {
             print a[x], typeof(a[x])
        }
   }

   function str_num_cmp(i1, v1, i2, v2) {
        str_num_to_a(i1, a1)
        str_num_to_a(i2, a2)
        if(a1[1] < a2[1])
            return -1
        if(a1[1] == a2[1]) {
            if(a1[2] < a2[2])
                return -1
            if(a1[2] == a2[2])
                return 0
        }
        return 1
   }

   BEGIN {
        keys["foo6"]
        keys["foo10"]
        #keys["\"foo6\""]
        #keys["\"foo10\""]
        PROCINFO["sorted_in"] = "str_num_cmp"
        for (k in keys) {
            print k, keys[k]
        }
   }

The above code works fine BUT then try uncommenting the 2 `#keys` lines 
in the BEGIN section (doesn't matter if you comment out the other 2 keys 
lines or not) and you'll get:

   $ awk -f tst.awk
   awk: tst.awk:17: fatal: internal error line 1349, file:
   /home/corinna/tmp/gawk-5.1.1/gawk-5.1.1-1.x86_64/src/gawk-5.1.1/eval.c

Line 17 is the "return 0" in the middle function. Now comment out the 
loop in the top function and the code will work again. I tried to reduce 
this example further (it's already reduced from the original) but after 
a lot of trial and error I surrendered.

    Ed.