CHECK-NEXT behavior deviating from FileCheck

[frederik-h]

I can run most tests from my testsuite that uses lit and FileCheck with FileCheck replaced by filecheck.
But I have an example input for which the behavior of filecheck deviates from the behavior of FileCheck from LLVM 17 (more concretely, the build from the Ubuntu 22.04 package).
Consider a file output with the following content

ax
x, x,
x,

and a file check-input with the following content

//CHECK: ax
//CHECK-NEXT: x,  x,
//CHECK: x,

Running cat output | FileCheck-17 input succeeds without output and with exit code 0.
With filecheck, I get this result:

$ cat output |  filecheck input
input:3: error: Couldn't match "x,".
Current position at <stdin>:3:1

^

The error goes away if I turn the CHECK-NEXT into a CHECK or if I remove a , at the end of one of the two lines (in both files).

[AntonLydike]

Very interesting find! I hadn't noticed it, but there does appear to be a bug in the whitespace matching logic! Fixing it!