[fpc-pascal] Generic type conflicts

[Ben Grasset]

On Thu, Nov 7, 2019 at 3:03 AM Sven Barth via fpc-pascal <
fpc-pascal at lists.freepascal.org> wrote:

> If there is no type checking, then it is *not* verified by the compiler.
>

Perhaps "no type checking" was not the write way to put it. A better way to
describe it might be:

Since the compiler *always* knows exactly which type a generic type
constraint currently amounts to, it is clearly capable of properly choosing
specific code paths based on that, if given some kind of
constant-evaluatable boolean condition that is specifically based on
TTypeKind.

In general, it's *exactly* the same concept as something like using SizeOf
in an {$IF} / {$ELSEIF} directive pair (where the compiler does indeed
completely ignore everything in the "false" section):

program Example;

begin
  // You can in fact put stuff that isn't even close to valid code in
whichever block is false for you, below.
  {$IF SizeOf(Pointer) = 4}
    WriteLn('The size of a pointer is 4 bytes.');
  {$ELSEIF SizeOf(Pointer) = 8}
    WriteLn('The size of a pointer is 8 bytes.');
  {$ENDIF}
end.

Not that it has to do precisely the same thing for a hypothetical
TTypeKind-based choice.

E.G. it could still do the full checking, as the only thing that really
matters is that it does not actively raise error messages only relevant for
Integer under code blocks only entered for tkFloat.
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