[Accepted as a DR at the July, 2019 meeting.]

A lambda expression in two translation units has distinct closure types, because each such expression's type is unique within the program. This results in an issue with the ODR, which requires that the definitions of an entity are identical. For example, if

  template <int> void f() {std::function<void()> f = []{};}

appears in two translation units, different specializations of function's constructor template are called, which violates 6.3 [basic.def.odr] bullet 6.4.

Issue 765 dealt with a similar problem for inline functions, but the issue still remains for templates.