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

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().

That doesn't sound like a valid wndproc

It certainly is,

It looks like you missed the part where you "move the context into the lambda body via capture specifier."