function construction - Another difference between Set and Setdelayed. Evaluation shortcut?

A little while ago I wondered why

f[x_] = f[x]

gives an infinite iteration. I ended up discovering the following difference in evaluation between Set and SetDelayed with Evaluate.

count = 0;
ClearAll@a
a /; (count++ < 20) = {a}
a // OwnValues
count = 0;
a

Output

{{{{{{{{{{{{{{{{{{{{{a}}}}}}}}}}}}}}}}}}}}}
{HoldPattern[a /; count++ < 20] :> {a}}
{a}

and

count = 0;
ClearAll@b
b /; (count++ < 20) := Evaluate@{b}

b // OwnValues
count = 0;
b

Output

{HoldPattern[b /; count++ < 20] :> {b}}
{{{{{{{{{{{{{{{{{{{{b}}}}}}}}}}}}}}}}}}}}

Can somebody explain the difference? Can we say that there is an evaluation shortcut at work here?

Related

This is a follow up question: Strange results of definitions using OwnValues

Why x = x doesn't cause an infinite loop, but f[x_] := f[x] does?

Does Set vs. SetDelayed have any effect after the definition was done?

Answer

I thought to give a bit more insight into why Update is needed, as pointed out in the other answers. Its documentation says Update may be needed when a change in 1 symbol changes another via a condition test.

In Jacob's example, setting count = 0 changes the condition test outcome, and thus a or b on the LHS. Consequently, a or b on the RHS is supposed to change. However, RHS a equals the old LHS a, which was undefined because count>=20, and needs Update to be changed. RHS b behaves the same, but was not evaluated in SetDelayed because Evaluate occurs before SetDelayed, so count is unchanged, and RHS b evaluates to LHS b with count<20. If we now reset count=0, evaluating b will return {b}.

To illustrate, I modify the example to separate LHS and RHS. MMA is clever enough to automatically update LHS declared as a variable, so I have to make a function:

count=0;
ClearAll[LHS,RHS];
LHS[]/;(count++<20)={RHS};

RHS=Unevaluated@LHS[];

count=0;
RHS (* Equals LHS[] with count >= 20 *)

(* Tell Wolfram Language about changes affecting RHS which depends on LHS *)
Update@Unevaluated@LHS;
RHS

LHS[]

{{{{{{{{{{{{{{{{{{{{LHS[]}}}}}}}}}}}}}}}}}}}}