Software like Blackduck or Scanoss is designed to identify exactly that type of behaviour. It is used very often to scan closed source software and to check whether it contains snippets that are copied from open source with incompatible licenses (e.g. GPL).
To be able to do so, these softwares build a syntax tree of what your code snippet is, and compare the tree structure with similar trees in open source software without being fooled by variable names. To speed up the search, they also compute a signature for these trees so that the signature can be more easily searched in their database of open source code.
And that's all well and good, but that code that asserts to be protected by GPL still has to stand the abstraction-filtration-comparison test.
The plain fact is that you can claim copyright on plenty of stuff that isn't copyrightable.
Consider AI model weights at all: they're the result of an automatic process and contain no human expression; almost by definition, model weights shouldn't be copyrightable, but people are still releasing "open source" models with supposed licenses.
But there has to be a threshold. If a GPL project contains a function which takes two variables and returns x+y, and I have functionally identical code in a project I made with an incompatible license, it is obviously absurd to sue me.
You are right but there is no legally defined threshold so it's subjective.
As a matter of fact, the Eclipse Foundation requires every contributor to declare that every piece of code is their own original creation and is not a copy/paste from other projects, with the exception possibly of other Eclipse Foundation or Apache Foundation projects because their respective licenses allow that. Even code snippets from StackOverflow are formally forbidden.
If I am not mistaken, in the Oracle-Google trial over Java on Android, at the end Google re-implementation of Java API on Android was considered fair-use, because Google kept the original "signatures" of the Java SDK API and rewrote most of the implementation with the exception of copying "0.4% of the total Java source code and was minimal" [1]
However the trial came to this conclusion after several iterations in court.
You're right, there is. The threshold is whatever a court decides is "substantial similarity" in that particular case. But there's no way to know that ahead of time as the interpretation/decision is subjective.
To be able to do so, these softwares build a syntax tree of what your code snippet is, and compare the tree structure with similar trees in open source software without being fooled by variable names. To speed up the search, they also compute a signature for these trees so that the signature can be more easily searched in their database of open source code.