Fix variable_ref_type for unsupported types and GenericNonlinearExpr

src/nlp_expr.jl

@@ -133,7 +133,7 @@ Alias for `GenericNonlinearExpr{VariableRef}`, the specific
 """
 const NonlinearExpr = GenericNonlinearExpr{VariableRef}
 
-variable_ref_type(::GenericNonlinearExpr{V}) where {V} = V
+variable_ref_type(::Type{GenericNonlinearExpr{V}}) where {V} = V
 
 const _PREFIX_OPERATORS =
     (:+, :-, :*, :/, :^, :||, :&&, :>, :<, :(<=), :(>=), :(==))

src/variables.jl

@@ -224,6 +224,14 @@ variable type associated with the model or expression type `F`.
 """
 variable_ref_type(::F) where {F} = variable_ref_type(F)
 
+function variable_ref_type(::Type{F}) where {F}
+    return error(
+        "Unable to compute the `variable_ref_type` of the type `$F`. If you " *
+        "are developing a JuMP extension, define a new method for " *
+        "`JuMP.variable_ref_type(::Type{$F})`",
+    )
+end
+
 variable_ref_type(::Type{V}) where {V<:AbstractVariableRef} = V
 
 value_type(::Type{<:AbstractVariableRef}) = Float64

test/test_nlp_expr.jl

@@ -971,4 +971,11 @@ function test_operator_max()
     return
 end
 
+function test_variable_ref_type()
+    for V in (GenericVariableRef{Int}, VariableRef)
+        @test variable_ref_type(GenericNonlinearExpr{V}) == V
+    end
+    return
+end
+
 end  # module

test/test_variable.jl

@@ -1555,4 +1555,18 @@ function test_parameter_arrays()
     return
 end
 
+function test_variable_ref_type_unsupported()
+    for F in (Vector{VariableRef}, Vector{Int})
+        @test_throws(
+            ErrorException(
+                "Unable to compute the `variable_ref_type` of the type `$F`. If you " *
+                "are developing a JuMP extension, define a new method for " *
+                "`JuMP.variable_ref_type(::Type{$F})`",
+            ),
+            variable_ref_type(F),
+        )
+    end
+    return
+end
+
 end  # module TestVariable