[rovingeye]

I assume by "move the context into the body" you mean using GetWindowLongPtr? Why not just use a static wndproc at that point?

[pjmlp]

I mean using a static C++ lambda that moves the context into the lambda body via capture specifier.

C++ lambdas are basically old style C++ functors that are compiled generated, with the calling address being the operator().

[rovingeye]

That doesn't sound like a valid wndproc