Issue with time derivatives on upper spatial boundary

[xtalax]

https://discourse.julialang.org/t/space-varying-parameter-in-pde-with-modelingtoolkit/91897/5

[touste]

Hi, I put up a simple minimal example highlighting this issue:

using OrdinaryDiffEq, ModelingToolkit, MethodOfLines, DomainSets

# Parameters, variables, and derivatives
@parameters t x
@variables u(..)
Dt = Differential(t)
Dxx = Differential(x)^2

# 1D PDE and boundary conditions
eq  = Dt(u(t, x)) ~ Dxx(u(t, x))
bcs = [u(0, x) ~ 20,
        Dt(u(t, 0)) ~ 100, # Heat source
        Dt(u(t, 1)) ~ 0] # Zero flux

# Space and time domains
domains = [t ∈ Interval(0.0, 1.0),
           x ∈ Interval(0.0, 1.0)]

# PDE system
@named pdesys = PDESystem(eq, bcs, domains, [t, x], [u(t, x)])

# Method of lines discretization
dx = 0.1
order = 2
discretization = MOLFiniteDifference([x => dx], t)

# Convert the PDE problem into an ODE problem
prob = discretize(pdesys,discretization)

# Solve ODE problem
sol = solve(prob, Rodas4(), saveat=0.2)

discrete_x = sol[x]
discrete_t = sol[t]
solu = sol[u(t, x)] # Temperature should increase with time

[xtalax]

The coveted 1 char fix