Prof. Stroustrup is very clear that any type that is not built-in is user-defined. See the second paragraph of section 9.1 in Programming Principles and Practice Using C++.
He even specifically calls out “standard library types” as an example of user-defined types. In other words, a user-defined type is any compound type.
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The article explicitly mentions that not everyone seems to agree, but this is IMHO mostly wishful thinking and not what the standard (and Prof. Stroustrup) are actually saying, only what some people want to read into it.