[Utilities] fix use of recursion in canonicalize!(::ScalarNonlinearFunction)

src/Utilities/functions.jl

@@ -1072,9 +1072,17 @@ function canonicalize!(
 end
 
 function canonicalize!(f::MOI.ScalarNonlinearFunction)
-    for (i, arg) in enumerate(f.args)
-        if !is_canonical(arg)
-            f.args[i] = canonicalize!(arg)
+    # Don't use recursion here. This gets called for all scalar nonlinear
+    # constraints.
+    stack = MOI.ScalarNonlinearFunction[f]
+    while !isempty(stack)
+        g = pop!(stack)
+        for (i, arg) in enumerate(g.args)
+            if arg isa MOI.ScalarNonlinearFunction
+                push!(stack, arg)
+            elseif !is_canonical(arg)
+                g.args[i] = canonicalize!(arg)
+            end
         end
     end
     return f

test/Utilities/test_functions.jl

@@ -2044,6 +2044,19 @@ function test_ScalarNonlinearFunction_is_canonical()
         f = MOI.ScalarNonlinearFunction(:sin, Any[f])
     end
     @test MOI.Utilities.is_canonical(f)
+    @test MOI.Utilities.canonicalize!(f) isa MOI.ScalarNonlinearFunction
+    g = MOI.ScalarNonlinearFunction(:^, Any[1.0 * x + 1.0 * x, 2])
+    for _ in 1:100_000
+        g = MOI.ScalarNonlinearFunction(:sin, Any[g])
+    end
+    @test !MOI.Utilities.is_canonical(g)
+    MOI.Utilities.canonicalize!(g)
+    h = MOI.ScalarNonlinearFunction(:^, Any[2.0 * x, 2])
+    for _ in 1:100_000
+        h = MOI.ScalarNonlinearFunction(:sin, Any[h])
+    end
+    @test MOI.Utilities.is_canonical(h)
+    @test isapprox(g, h)
     return
 end