What is the behavior when there are inappropriate types between the extern declaration and the definition?

Suppose I have two files:

== File1 ==

extern char* foo;

== File2 ==

double foo;

These two files seem to be compiling and are very well linked with g ++ and clang ++, despite the type mismatch. As far as I understand, the recommended practice is to put the extern declaration in the header, which both files include, so that File2 will cause an override error.

My questions:

• Does this lead to undefined behavior according to the C ++ standard? If not what happens in foo in File1?
• Can linkers catch this type of type mismatch?