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Why don't I obtain that $x$ belongs to the Reals?

FunctionDomain[x^2 + 4, x]
(* Out: True *)

FunctionDomain[x^2 + 4, x, Reals]
(* Out: True *)

However, there is no problem here:

FunctionDomain[Log[x]/Sqrt[x + y], x]
(* Out: x > 0 && x + y > 0 *) 
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  • $\begingroup$ True its mean in this case x \[Element] Interval[{-Infinity, Infinity}]. $\endgroup$ – Mariusz Iwaniuk Jun 21 '18 at 19:18
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FunctionDomain tells you quantitatively what the domain is:

FunctionDomain[Sqrt[x], x]
(* x >= 0 *)

True just means "no limits" here.

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