Add set_start_value(x, ::Nothing) (#2551)

src/variables.jl

@@ -898,24 +898,22 @@ function start_value(v::VariableRef)::Union{Nothing,Float64}
 end
 
 """
-    set_start_value(variable::VariableRef, value::Number)
+    set_start_value(variable::VariableRef, value::Union{Real,Nothing})
 
-Set the start value (MOI attribute `VariablePrimalStart`) of the variable `v` to
+Set the start value (MOI attribute `VariablePrimalStart`) of the `variable` to
 `value`.
 
+Pass `nothing` to unset the start value.
+
 Note: `VariablePrimalStart`s are sometimes called "MIP-starts" or "warmstarts".
 
 See also [`start_value`](@ref).
 """
-function set_start_value(variable::VariableRef, value::Number)
-    MOI.set(
-        owner_model(variable),
-        MOI.VariablePrimalStart(),
-        variable,
-        Float64(value),
-    )
+function set_start_value(variable::VariableRef, value::Union{Nothing,Float64})
+    MOI.set(owner_model(variable), MOI.VariablePrimalStart(), variable, value)
     return
 end
+set_start_value(x::VariableRef, v::Number) = set_start_value(x, Float64(v))
 
 """
     value(v::VariableRef; result = 1)

test/variable.jl

@@ -922,6 +922,18 @@ function test_Model_relax_integrality_error_cases(::Any, ::Any)
     @test_throws err relax_integrality(model)
 end
 
+function test_start_value(::Any, ::Any)
+    model = Model()
+    @variable(model, x)
+    @test start_value(x) === nothing
+    set_start_value(x, 1.0)
+    @test start_value(x) == 1.0
+    set_start_value(x, nothing)
+    @test start_value(x) === nothing
+    set_start_value(x, 1)
+    @test start_value(x) == 1.0
+end
+
 function runtests()
     for name in names(@__MODULE__; all = true)
         if !startswith("$(name)", "test_")