Why cannot take address to a nested local function in 64 bit Delphi?

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This trick was never officially supported by the language and you have been getting away with it to date due to the implementation specifics of the 32 bit compiler. The documentation is clear:

Nested procedures and functions (routines declared within other routines) cannot be used as procedural values.

If I recall correctly, an extra, hidden, parameter is passed to nested functions with the pointer to the enclosing stack frame. This is omitted in 32 bit code if no reference is made to the enclosing environment. In 64 bit code the extra parameter is always passed.

Of course a big part of the problem is that the Windows unit uses untyped procedure types for its callback parameters. If typed procedures were used the compiler could reject your code. In fact I view this as justification for the belief that the trick you used was never legal. With typed callbacks a nested procedure can never be used, even in the 32 bit compiler.

Anyway, the bottom line is that you cannot pass a nested function as parameter to another function in the 64 bit compiler.

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