Merge pull request #869 from Keno/kf/emptymm

Allow empty mass matrix for empty u0

Author: ChrisRackauckas

src/solve.jl

@@ -1125,10 +1125,11 @@ function get_concrete_u0(prob, isadapt, t0, kwargs)
         throw(IncompatibleInitialConditionError())
     end
 
+    nu0 = length(something(_u0, ()))
     if isdefined(prob.f, :mass_matrix) && prob.f.mass_matrix !== nothing &&
        prob.f.mass_matrix isa AbstractArray &&
-       size(prob.f.mass_matrix, 1) !== length(_u0)
-        throw(IncompatibleMassMatrixError(size(prob.f.mass_matrix, 1), length(_u0)))
+       size(prob.f.mass_matrix, 1) !== nu0
+        throw(IncompatibleMassMatrixError(size(prob.f.mass_matrix, 1), nu0))
     end
 
     if _u0 isa Tuple

test/downstream/solve_error_handling.jl

@@ -37,7 +37,14 @@ prob = ODEProblem{false}(f, (1.0, 1.0f0), tspan)
 prob = ODEProblem{false}(f, u0, (0.0 + im, 1.0))
 @test_throws DiffEqBase.ComplexTspanError solve(prob, Tsit5())
 
-u0 = [0.0, 0.0]
-fmm = ODEFunction(f, mass_matrix = zeros(3, 3))
-prob = ODEProblem(fmm, u0, (0.0, 1.0))
-@test_throws DiffEqBase.IncompatibleMassMatrixError solve(prob, Tsit5())
+for u0 in ([0.0, 0.0], nothing)
+    fmm = ODEFunction(f, mass_matrix = zeros(3, 3))
+    prob = ODEProblem(fmm, u0, (0.0, 1.0))
+    @test_throws DiffEqBase.IncompatibleMassMatrixError solve(prob, Tsit5())
+end
+
+# Allow empty mass matrix for empty u0
+fmm = ODEFunction((du, u, t)->nothing, mass_matrix = zeros(0, 0))
+prob = ODEProblem(fmm, nothing, (0., 1.))
+sol = solve(prob, Tsit5())
+@test isa(sol, DiffEqBase.ODESolution)