Substituting a constant = 1 seems to mutate variable type of certain types of parameters

[vyudu]

I'm really not sure what's going on here, but the variable type of Initial(p) seems to be silently changed to variable instead of parameter when substituting h = 1. This doesn't happen when any number other than 1 is substituted, or for normal parameters.

@constants h = 1
@variables x
eq = x ~ h * Initial(x)
dvs = Set(); ps = Set()
ModelingToolkit.collect_vars!(dvs, ps, eq, nothing; op = Initial) # dvs = Set(x), ps = Set(Initial(x))
empty!(dvs); empty!(ps)

eq = Symbolics.substitute(eq, h => 1) # eq = x ~ Initial(x)
ModelingToolkit.collect_vars!(dvs, ps, eq, nothing; op = Initial) # dvs = Set(x, Initial(x)), ps = Set()
ModelingToolkit.isparameter(eq.rhs) # false

on 9.65.0

[AayushSabharwal]

This isn't hashconsing, it's a bug in substitute. When substituting h => 1, it ends up in a state where

expr = h * Initial(x)
op = *
args = [1, Initial(x)] # values after substitution into `expr`

It then calls maketerm(typeof(expr), op, args, metadata(expr)). The problem is that this turns 1 * Initial(x) into Initial(x), then sets the metadata of the result to metadata(expr), which erases the parameter metadata from Initial(x).

[vyudu]

Thanks so much Aayush!