Merge pull request #3454 from ashutosh-b-b/bb/build_explicit_observed_function

Fix: Make `build explicit observed function` return Expr when `expression=true`

Author: ChrisRackauckas

src/systems/diffeqs/odesystem.jl

@@ -574,12 +574,18 @@ function build_explicit_observed_function(sys, ts;
         sys, ts, args...; p_start, p_end, filter_observed = obsfilter,
         output_type, mkarray, try_namespaced = true, expression = Val{true})
     if fns isa Tuple
+        if expression
+            return return_inplace ? fns : fns[1]
+        end
         oop, iip = eval_or_rgf.(fns; eval_expression, eval_module)
         f = GeneratedFunctionWrapper{(
             p_start + is_dde(sys), length(args) - length(ps) + 1 + is_dde(sys), is_split(sys))}(
             oop, iip)
         return return_inplace ? (f, f) : f
     else
+        if expression
+            return fns
+        end
         f = eval_or_rgf(fns; eval_expression, eval_module)
         f = GeneratedFunctionWrapper{(
             p_start + is_dde(sys), length(args) - length(ps) + 1 + is_dde(sys), is_split(sys))}(

test/odesystem.jl

@@ -1724,3 +1724,15 @@ end
     @mtkbuild ode = ODESystem(D(x(t)) ~ mat * x(t), t; constraints = cons)
     @test length(constraints(ModelingToolkit.get_constraintsystem(ode))) == 5
 end
+
+@testset "`build_explicit_observed_function` with `expression = true` returns `Expr`" begin
+    @variables x(t)
+    @mtkbuild sys = ODESystem(D(x) ~ 2x, t)
+    obsfn_expr = ModelingToolkit.build_explicit_observed_function(
+        sys, 2x + 1, expression = true)
+    @test obsfn_expr isa Expr
+    obsfn_expr_oop, obsfn_expr_iip = ModelingToolkit.build_explicit_observed_function(
+        sys, [x + 1, x + 2, x + t], return_inplace = true, expression = true)
+    @test obsfn_expr_oop isa Expr
+    @test obsfn_expr_iip isa Expr
+end