Fix error message if NonlinearExpression is mixed with new NL API

src/nlp_expr.jl

@@ -87,6 +87,7 @@ struct GenericNonlinearExpr{V<:AbstractVariableRef} <: AbstractJuMPScalar
     ) where {V<:AbstractVariableRef}
         for arg in args
             _throw_if_not_real(arg)
+            _throw_if_legacy(arg)
         end
         return new{V}(head, Any[a for a in args])
     end
@@ -97,11 +98,28 @@ struct GenericNonlinearExpr{V<:AbstractVariableRef} <: AbstractJuMPScalar
     ) where {V<:AbstractVariableRef}
         for arg in args
             _throw_if_not_real(arg)
+            _throw_if_legacy(arg)
         end
         return new{V}(head, args)
     end
 end
 
+_throw_if_legacy(::Any) = nothing
+
+function _throw_if_legacy(arg::Union{NonlinearExpression,NonlinearParameter})
+    return error(
+        "Cannot create a nonlinear expression that mixes features from " *
+        "both the legacy (macros beginning with `@NL`) and new " *
+        "(`NonlinearExpr`) nonlinear interfaces. You must use one or " *
+        "the other. Got: $arg",
+    )
+end
+
+# This method is necessary to catch errors in an expression like p * x where
+# p is a nonlinear parameter.
+variable_ref_type(::NonlinearParameter) = GenericVariableRef{Float64}
+variable_ref_type(::NonlinearExpression) = GenericVariableRef{Float64}
+
 variable_ref_type(::Type{GenericNonlinearExpr}, ::Any) = nothing
 
 function variable_ref_type(::Type{GenericNonlinearExpr}, x::AbstractJuMPScalar)

test/test_nlp_expr.jl

@@ -1092,4 +1092,40 @@ function test_error_nonlinear_expr_complex_constructor()
     return
 end
 
+function test_error_legacy_expression_constructor()
+    model = Model()
+    @variable(model, x)
+    @NLexpression(model, arg, x^3)
+    err = ErrorException(
+        "Cannot create a nonlinear expression that mixes features from " *
+        "both the legacy (macros beginning with `@NL`) and new " *
+        "(`NonlinearExpr`) nonlinear interfaces. You must use one or " *
+        "the other. Got: $arg",
+    )
+    @test_throws err GenericNonlinearExpr(:*, Any[x, arg])
+    @test_throws err GenericNonlinearExpr(:*, x, arg)
+    @test_throws err (x * arg)
+    @test_throws err (arg * x)
+    return
+end
+
+function test_error_legacy_parameter_constructor()
+    model = Model()
+    @variable(model, x)
+    @NLparameter(model, p == 1)
+    err = ErrorException(
+        "Cannot create a nonlinear expression that mixes features from " *
+        "both the legacy (macros beginning with `@NL`) and new " *
+        "(`NonlinearExpr`) nonlinear interfaces. You must use one or " *
+        "the other. Got: $p",
+    )
+    @test_throws err GenericNonlinearExpr(:*, Any[x, p])
+    @test_throws err GenericNonlinearExpr(:*, Any[p, x])
+    @test_throws err GenericNonlinearExpr(:*, x, p)
+    @test_throws err GenericNonlinearExpr(:*, p, x)
+    @test_throws err (x * p)
+    @test_throws err (p * x)
+    return
+end
+
 end  # module