Undefined symbols for constexpr function

Question

When I attempt compiling the following code I get a linker error: Undefined symbols for architecture x86_64: \"Foo()\", referenced from: _main in main.o using L

Answer 1

Why does declaring the function constexpr cause a linker error?

That is because constexpr functions are implicitly inline. Per Paragraph 7.1.5/2 of the C++11 Standard:

A constexpr specifier used in the declaration of a function that is not a constructor declares that function to be a constexpr function. Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor. constexpr functions and constexpr constructors are implicitly inline (7.1.2).

Per Paragraph 7.1.2/4, then:

An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case (3.2). [...]