Question

For foo declared as Foo& foo = ...;, is there any difference between capture-by-value and capture-by-reference semantics for lambdas?

Answer 1

Yes, there is a difference.

§5.1.2 [expr.prim.lambda] p14

An entity is captured by copy if it is implicitly captured and the capture-default is = or if it is explicitly captured with a capture that does not include an &. For each entity captured by copy, an unnamed nonstatic data member is declared in the closure type. The declaration order of these members is unspecified. The type of such a data member is the type of the corresponding captured entity if the entity is not a reference to an object, or the referenced type otherwise.

So, if you capture an identifier that names a reference by value, you get a copy of the referenced object.