https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117490
--- Comment #2 from keinfluegeltier at posteo dot de --- The aliasing rule in §6.5.1 does not forbid the access if the lvalue used has "_a type compatible with the effective type of the object_". The effective type of `x` is the declared one, which is the type `s3` in the second translation unit and is specified to be compatible with both `s1` and `s2_alt` that are used to access `x`. So the call can't have undefined behavior by §6.5.1. I do not see anything else that could cause the call to have undefined behavior. (But I agree that it seems weird that compatibility doesn't form an equivalence relation among structures.)
