Allow constrained MIRK without f_prototype

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

lib/BoundaryValueDiffEqMIRK/src/mirk.jl

@@ -84,7 +84,11 @@ function SciMLBase.__init(
     y = __alloc.(copy.(y₀.u))
     TU, ITU = constructMIRK(alg, T)
     stage = alg_stage(alg)
-    f_prototype = isnothing(prob.f.f_prototype) ? nothing : __vec(prob.f.f_prototype)
+    f_prototype = if isnothing(prob.f.f_prototype)
+        constraint ? __vec(u0) : nothing
+    else
+        __vec(prob.f.f_prototype)
+    end
     L_f_prototype = isnothing(f_prototype) ? N : length(f_prototype)
 
     k_discrete = if !constraint

lib/BoundaryValueDiffEqMIRK/test/Core/dynamic_optimization_tests.jl

@@ -78,12 +78,16 @@ end
         res[2] = u(1.0)[1]
     end
 
-    constrained_fun = BVPFunction(constrained_f!, constrained_bc!; f_prototype = zeros(2))
-    constrained_prob = BVProblem(
-        constrained_fun, [0.0, 0.0], (0.0, 1.0); lb = [-10.0, -10.0], ub = [10.0, 10.0]
-    )
-    sol = solve(constrained_prob, MIRK4(; optimize = IpoptOptimizer()); dt = 0.01)
+    for constrained_fun in (
+            BVPFunction(constrained_f!, constrained_bc!),
+            BVPFunction(constrained_f!, constrained_bc!; f_prototype = zeros(2)),
+        )
+        constrained_prob = BVProblem(
+            constrained_fun, [0.0, 0.0], (0.0, 1.0); lb = [-10.0, -10.0], ub = [10.0, 10.0]
+        )
+        sol = solve(constrained_prob, MIRK4(; optimize = IpoptOptimizer()); dt = 0.01)
 
-    @test SciMLBase.successful_retcode(sol)
-    @test sol(0.5) ≈ [sinh(0.5) / sinh(1.0), -cosh(0.5) / sinh(1.0)] rtol = 1.0e-4
+        @test SciMLBase.successful_retcode(sol)
+        @test sol(0.5) ≈ [sinh(0.5) / sinh(1.0), -cosh(0.5) / sinh(1.0)] rtol = 1.0e-4
+    end
 end