fix: pin the unassigned type parameter of the observed dependency graph

lib/ModelingToolkitBase/src/utils.jl

@@ -1201,14 +1201,27 @@ function is_numeric_symtype(T::Type)
     return T <: Number || T <: AbstractArray && is_numeric_symtype(eltype(T))
 end
 
+"""
+The concrete type of the graph returned by [`observed_dependency_graph`](@ref).
+"""
+const ObservedDependencyGraphT = DiCMOBiGraph{
+    false, Int, BipartiteGraph{Int, Nothing},
+    Matching{Unassigned, Vector{Union{Unassigned, Int}}},
+}
+
 """
     $(TYPEDSIGNATURES)
 
 Return the `DiCMOBiGraph` denoting the dependencies between observed equations `eqs`.
 """
-function observed_dependency_graph(sys::AbstractSystem, eqs::Vector{Equation})
+function observed_dependency_graph(
+        sys::AbstractSystem, eqs::Vector{Equation}
+    )::ObservedDependencyGraphT
     graph, assigns = observed2graph(sys, eqs, getproperty.(eqs, (:lhs,)))
-    matching = complete(Matching(Vector{Union{Unassigned, Int}}(assigns)))
+    # The unassigned type parameter is given explicitly instead of letting `Matching`
+    # infer it from the eltype, since that inference is not part of a stable contract
+    # and has differed between `BipartiteGraphs` versions.
+    matching = complete(Matching{Unassigned}(Vector{Union{Unassigned, Int}}(assigns)))
     return DiCMOBiGraph{false}(graph, matching)
 end
 
@@ -1219,10 +1232,7 @@ function should_invalidate_mutable_cache_entry(::Type{ObservedGraphCacheKey}, pa
 end
 
 struct ObservedGraphCache
-    graph::DiCMOBiGraph{
-        false, Int, BipartiteGraph{Int, Nothing},
-        Matching{Unassigned, Vector{Union{Unassigned, Int}}},
-    }
+    graph::ObservedDependencyGraphT
     obsvar_to_idx::Dict{Any, Int}
 end
 

lib/ModelingToolkitBase/test/code_generation.jl

@@ -158,3 +158,14 @@ end
     @mtkcomplete sys = System([D(y) ~ 2y + sum(x)], t, [y], []; observed = [x ~ [y, y + 1, y + 2]])
     @test ModelingToolkitBase.observed_equations_used_by(sys, [x[1]]) == [1]
 end
+
+@testset "`observed_dependency_graph` result is cacheable" begin
+    @variables x(t) y(t) z(t)
+    @mtkcompile sys = System([D(x) ~ z, y ~ 2x + 1, z ~ 3y], t)
+    obs = ModelingToolkitBase.observed(sys)
+    graph = ModelingToolkitBase.observed_dependency_graph(sys, obs)
+    @test graph isa fieldtype(ModelingToolkitBase.ObservedGraphCache, :graph)
+    # this populates the observed graph cache, which requires the above type to match
+    @test ModelingToolkitBase.observed_equations_used_by(sys, [equations(sys)[1].rhs]) ==
+        [1, 2]
+end