Are they the same variable, or not? In other words, what is
If it is external, then no. If it is internal, then it's OK unless the two definitions both occur in the same file.
If there is no linkage, then there is no problem.
Unless I've overlooked something,
thread_local has no impact on linkage, so the usual rules apply (and defining the variable
thread_local in one translation unit, and not in another, is a violation of the one-definition rule).
I think there's a bug in the standard here, however. The
standard (§7.1.1/1) says that "If thread_local appears in any
declaration of a variable it shall be present in all
declarations of that entity." There's no explicit statement
that a diagnostic is not required, or that violation of this
rule is undefined behavior, so a compiler is required to
diagnose the error. Except that, of course, if you define at
thread_local int i;
in one translation unit, and:
in another, then the compiler probably can't diagnose the error
(and I'm fairly sure the committee didn't want to require it).
My guess is that the intent here is undefined behavior.