Answer by Brian Bi for Is accessing static class member via unitilialized pointer UB?

The semantics of a->n are that *a is evaluated, but not accessed, since the data member is static. See C++17 [expr.ref]:

... The postfix expression before the dot or arrow is evaluated ... The expressionE1->E2 is converted to the equivalent form (*(E1)).E2 ...

There is also a footnote that says:

If the class member access expression is evaluated, the subexpression evaluation happens even if the result is unnecessary to determine the value of the entire postfix expression, for example if the id-expression denotes a static member.

In this case, the expression *a is evaluated. Since a is an automatic variable that has not been initialized, [dcl.init]/12 applies:

If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases: [ ... ]

Evaluating *a clearly requires accessing the value of the pointer variable a, which is an indeterminate value, therefore it is UB.