Why does it occur error: type of expression is ambiguous without more context?

I am studying the swift programming language.

Now I'm looking at how opaque types works.

Why does it occur error: type of expression is ambiguous without more context? (Swift 5.1)

protocol NumericExpression {
    associatedtype TNumeric: Numeric
    var value: TNumeric { get }
}

struct Expression { 
  
    static func sum<TA: NumericExpression, TB: NumericExpression>(_ a: TA,_ b: TB) -> some NumericExpression
        where TA.TNumeric == TB.TNumeric {
        
        return NumericExpressionSum(a: a, b: b)
    } 
   
    static func multi<TA: NumericExpression, TB: NumericExpression>(_ a: TA, _ b: TB) -> some NumericExpression
        where TA.TNumeric == TB.TNumeric {
        
        return NumericExpressionMulti(a: a, b: b)
    }
}

extension Int: NumericExpression {
    var value: Self {
        return self
    }
}

struct NumericExpressionSum<TA: NumericExpression, TB: NumericExpression>: NumericExpression
    where TA.TNumeric == TB.TNumeric { 
 
    let a: TA
    let b: TB   
    var value: TA.TNumeric { a.value + b.value }
}

struct NumericExpressionMulti<TA: NumericExpression, TB: NumericExpression>: NumericExpression
    where TA.TNumeric == TB.TNumeric { 
   
    let a: TA
    let b: TB   
    var value: TA.TNumeric { a.value * b.value }
}

No compile error.

Sample code with compile error:

var s1 = Expression.sum(1, 2)
var s2 = Expression.sum(2, 3)

var m1 = Expression.multi(s1, s2)
var m2 = Expression.multi(3, 4)

print(m1.value) // 15
print(m2.value) // 12

print(type(of: m1)) // NumericExpressionMulti<NumericExpressionSum<Int, Int>, NumericExpressionSum<Int, Int>>
print(type(of: m2)) // NumericExpressionMulti<Int, Int>

print(type(of: m1).TNumeric) // Int
print(type(of: m2).TNumeric) // Int

let m3 = Expression.multi(m1, m2) //Error: type of expression is ambiguous without more context

Does this work for swift 5.5?

Thanks!