Fix printing when show called on invalid variable or constraint (#3763)

src/print.jl

@@ -736,6 +736,9 @@ julia> function_string(MIME("text/plain"), 2 * x + 1)
 ```
 """
 function function_string(mode::MIME"text/plain", v::AbstractVariableRef)
+    if !is_valid(owner_model(v), v)
+        return "InvalidVariableRef"
+    end
     var_name = name(v)
     if isempty(var_name)
         return anonymous_name(mode, v)
@@ -744,6 +747,9 @@ function function_string(mode::MIME"text/plain", v::AbstractVariableRef)
 end
 
 function function_string(mode::MIME"text/latex", v::AbstractVariableRef)
+    if !is_valid(owner_model(v), v)
+        return "InvalidVariableRef"
+    end
     var_name = name(v)
     if isempty(var_name)
         return anonymous_name(mode, v)
@@ -1038,6 +1044,9 @@ julia> constraint_string(MIME("text/plain"), c)
 ```
 """
 function constraint_string(mode::MIME, ref::ConstraintRef; in_math_mode = false)
+    if !is_valid(owner_model(ref), ref)
+        return "InvalidConstraintRef"
+    end
     return constraint_string(
         mode,
         name(ref),

test/test_print.jl

@@ -119,6 +119,13 @@ function JuMP.add_constraint(
     )
 end
 
+function JuMP.is_valid(
+    model::M,
+    con_ref::ConstraintRef{M,CustomIndex},
+) where {M<:GenericModel}
+    return 1 <= con_ref.index.value <= length(model.ext[:custom_constraints])
+end
+
 function JuMP.constraint_object(cref::ConstraintRef{Model,CustomIndex})
     return cref.model.ext[:custom_constraints][cref.index.value]
 end
@@ -1070,4 +1077,21 @@ function test_print_omit_vector()
     return
 end
 
+function test_invalid_references()
+    model = Model()
+    @variable(model, x[1:2])
+    @constraint(model, c[i in 1:2], x[i] <= i)
+    delete(model, c[1])
+    delete(model, x[1])
+    mime = MIME("text/plain")
+    @test sprint(show, mime, x[1]) == "InvalidVariableRef"
+    @test occursin("InvalidVariableRef", sprint(show, mime, x))
+    @test sprint(show, mime, c[1]) == "InvalidConstraintRef"
+    @test occursin("InvalidConstraintRef", sprint(show, mime, c))
+    mime = MIME("text/latex")
+    @test sprint(show, mime, x[1]) == "\$ InvalidVariableRef \$"
+    @test sprint(show, mime, c[1]) == "InvalidConstraintRef"
+    return
+end
+
 end  # TestPrint