Powershell trap not being triggered consistently

Question

I can't understand why I'm seeing this behavior in Powershell:

PS C:\> trap { "Got it!" } 1/0
Attempted to divide by zero.
At line:1 char:22
+ trap { "Got it!" } 1/0 <<<<

PS C:\> trap { "Got it!" } 1/$null
Got it!
Attempted to divide by zero.
At line:1 char:22
+ trap { "Got it!" } 1/$ <<<< null

Why does one expression trigger the trap and the other doesn't?

Answer 1

I would consider your first case to be a parse error. That is the parser is attempting to do constant folding (precomputing the value) and errors at that point because it gets a divide by zero exception. Other syntax errors behave the same way i.e. they don't trigger the trap:

trap { "Got it!" } 1/;
You must provide a value expression on the right-hand side of the '/' operator.

If you change the code to this:

$denom = 0
trap { "Got it!" } 1/$denom
Got it!
Attempted to divide by zero.

Then the trap fires because the parser can no longer precompute the value.