Lambdapp inserts #line directives into the source code, compilers are required to respect those directives and utilize them when producing debug sections in the binary. For instance in the case of gcc/clang on *nix the compiler will produce .debug_line sections as part of the DWARF debug format. Debuggers like gdb and even valgrind utilize this information to provide correct output. So to answer your question, a debugger would react to this code exactly how it should react, as if the lambda was called via a function and the file / line should be correct.