The assertion indicates a type confusion. In particular, setCallee stores a JSCell into a WriteBarrier<JSFunction> which is not actually a JSFunction, triggering this assertion.
Below is my preliminary analysis of the bug.
When DFG compiles v11, it decides to inline v23 and the isFinite function. The relevant parts of the resulting DFG graph (with many omissions) follow:
Note that some bytecode registers (locX) are reused to hold different values in this code.
The DFGPhantomInsertionPhase is responsible for identifying bytecode registers (locX) that have to be recovered during a bailout and placing Phantom nodes into the IR to ensure the required DFG values are alive so the bytecode registers can be restored from them. When the DFGPhantomInsertionPhase phase runs on this code and wants to determine the values needed for a bailout somewhere at the start of the try block, it decides that loc32 would have to be restored as it is assigned above but still used further down (in the inlined code of isFinite). As such, it inserts a Phantom node. When the bailout then actually happens (presumably because the `new Function()` fails), loc32 is attempted to be restored (by then, CreateDirectArguments has been replaced by a PhantomCreateDirectArguments which doesn't actually create the arguments object unless a bailout happens), resulting in a call to operationCreateDirectArgumentsDuringExit. This call requires the value of `callee` as argument. As such, the callee value is reconstructed as well. In the inlined callframe, the callee value is expected to be stored in loc30 (I think). However, by the time the bailout happens, loc30 has been reused, in this case by storing the global object into it. As such, the code that recovers the values (incorrectly) restores the callee value to the global object and passes it to operationCreateDirectArgumentsDuringExit. When this reference is then stored into a WriteBarrier<JSFunction> during a call to setCallee, an assertion is raised in debug builds. It is not clear to me at which point a different decision should have been made here.
Unfortunately, it is quite tedious to manually modify this sample as most changes to it will quickly break the specific bytecode register allocation outcome required to trigger the bug. I could imagine this bug to be exploitable if the invalid callee value is somehow subsequently accessed by code, e.g. user supplied code, the GC, or other parts of the engine that inspect bytecode registers, and assumed to be a JSFunction*. However, I have not verified that this is possible.
Javascriptcore type confusion during bailout when reconstructing arguments objects