Update error message of get_fallback

src/attributes.jl

@@ -295,29 +295,63 @@ end
 function get(model::ModelLike, attr::AnyAttribute, args...)
     return get_fallback(model, attr, args...)
 end
-function get_fallback(model::ModelLike, attr::AnyAttribute, args...)
+
+function get_fallback(
+    model::ModelLike,
+    attr::Union{AbstractModelAttribute,AbstractOptimizerAttribute},
+)
     return throw(
         ArgumentError(
-            "ModelLike of type $(typeof(model)) does not support accessing the attribute $attr",
+            "$(typeof(model)) does not support getting the attribute $(attr).",
         ),
     )
 end
 
-"""
-    get!(output, model::ModelLike, args...)
+function get_fallback(
+    model::ModelLike,
+    attr::AbstractVariableAttribute,
+    ::VariableIndex,
+)
+    return throw(
+        ArgumentError(
+            "$(typeof(model)) does not support getting the attribute $(attr).",
+        ),
+    )
+end
 
-An in-place version of `get`.
-The signature matches that of `get` except that the the result is placed in the vector `output`.
-"""
-function get! end
-function get!(output, model::ModelLike, attr::AnyAttribute, args...)
+function get_fallback(
+    model::ModelLike,
+    attr::AbstractConstraintAttribute,
+    ::ConstraintIndex,
+)
+    return throw(
+        ArgumentError(
+            "$(typeof(model)) does not support getting the attribute $(attr).",
+        ),
+    )
+end
+
+function get_fallback(::ModelLike, attr::AnyAttribute, args...)
     return throw(
         ArgumentError(
-            "ModelLike of type $(typeof(model)) does not support accessing the attribute $attr",
+            "Unable to get attribute $(attr): invalid arguments $(args).",
         ),
     )
 end
 
+"""
+    get!(output, model::ModelLike, args...)
+
+An in-place version of [`get`](@ref).
+
+The signature matches that of [`get`](@ref) except that the the result is placed
+in the vector `output`.
+"""
+function get!(output, model::ModelLike, attr::AnyAttribute, args...)
+    output .= get(model, attr, args...)
+    return
+end
+
 """
     set(optimizer::AbstractOptimizer, attr::AbstractOptimizerAttribute, value)
 

test/attributes.jl

@@ -140,6 +140,36 @@ function test_no_constraint_name()
     )
 end
 
+function test_get_fallback()
+    model = DummyModelWithAdd()
+    x = MOI.add_variable(model)
+    c = MOI.add_constraint(model, x, MOI.EqualTo(0.0))
+    err = ArgumentError(
+        "$(typeof(model)) does not support getting the attribute " *
+        "$(MOI.SolveTimeSec()).",
+    )
+    @test_throws(err, MOI.get(model, MOI.SolveTimeSec()))
+    output = Ref{Cdouble}()
+    @test_throws(err, MOI.get!(output, model, MOI.SolveTimeSec()))
+    errv = ArgumentError(
+        "$(typeof(model)) does not support getting the attribute " *
+        "$(MOI.VariablePrimal()).",
+    )
+    @test_throws(errv, MOI.get(model, MOI.VariablePrimal(), x))
+    errc = ArgumentError(
+        "$(typeof(model)) does not support getting the attribute " *
+        "$(MOI.ConstraintPrimal()).",
+    )
+    @test_throws(errc, MOI.get(model, MOI.ConstraintPrimal(), c))
+    @test_throws(
+        ArgumentError(
+            "Unable to get attribute $(MOI.VariablePrimal()): invalid " *
+            "arguments $((c,)).",
+        ),
+        MOI.get(model, MOI.VariablePrimal(), c),
+    )
+end
+
 function test_ConstraintBasisStatus_fallback()
     model = DummyModelWithAdd()
     c = MOI.ConstraintIndex{MOI.SingleVariable,MOI.EqualTo{Float64}}(1)