Fix missing value_type methods for expressions (#4002)

Author: odow

src/JuMP.jl

@@ -94,6 +94,15 @@ BigFloat
 """
 value_type(::Type{<:AbstractModel}) = Float64
 
+value_type(::Type{T}) where {T<:Number} = T
+
+function value_type(::Type{T}) where {T}
+    return error(
+        "Unable to compute the `value_type($T)`. If you are developing a " *
+        "JuMP extension, define a new method for `JuMP.value_type(::Type{$T})`",
+    )
+end
+
 mutable struct GenericModel{T<:Real} <: AbstractModel
     # In MANUAL and AUTOMATIC modes, CachingOptimizer.
     # In DIRECT mode, will hold an AbstractOptimizer.

src/aff_expr.jl

@@ -143,6 +143,8 @@ end
 
 variable_ref_type(::Type{GenericAffExpr{C,V}}) where {C,V} = V
 
+value_type(::Type{GenericAffExpr{C,V}}) where {C,V} = C
+
 function owner_model(x::GenericAffExpr)
     if !isempty(x.terms)
         return owner_model(first(keys(x.terms)))

src/nlp_expr.jl

@@ -112,6 +112,8 @@ function variable_ref_type(::Type{GenericNonlinearExpr}, x::AbstractJuMPScalar)
     return variable_ref_type(x)
 end
 
+value_type(::Type{GenericNonlinearExpr{V}}) where {V} = value_type(V)
+
 function _has_variable_ref_type(a)
     return variable_ref_type(GenericNonlinearExpr, a) !== nothing
 end

src/quad_expr.jl

@@ -87,6 +87,8 @@ end
 
 variable_ref_type(::Type{GenericQuadExpr{C,V}}) where {C,V} = V
 
+value_type(::Type{GenericQuadExpr{C,V}}) where {C,V} = C
+
 function owner_model(x::GenericQuadExpr)
     model = owner_model(x.aff)
     if model !== nothing

test/test_expr.jl

@@ -595,4 +595,26 @@ function test_issue_3982_core()
     return
 end
 
+function test_value_type()
+    for T in (Int, Float64, BigFloat)
+        model = GenericModel{T}()
+        @variable(model, x)
+        @test value_type(T) == T
+        @test value_type(typeof(model)) == T
+        @test value_type(typeof(x)) == T
+        @test value_type(typeof(one(T) * x)) == T
+        @test value_type(typeof(one(T) * x * x)) == T
+        @test value_type(typeof(log(x))) == T
+    end
+    T = Symbol
+    @test_throws(
+        ErrorException(
+            "Unable to compute the `value_type($T)`. If you are developing a " *
+            "JuMP extension, define a new method for `JuMP.value_type(::Type{$T})`",
+        ),
+        value_type(T),
+    )
+    return
+end
+
 end  # TestExpr