Bug Concatenation of non-empty String with value type causes a runtime exception

I'm getting an error when concatenating a non-empty string with a value type:
[libil2cpp] String concatenation for objects of type System.Object is not supported.

The behavior of concatenation is seemingly inconsistent, and I made 5 cases to test each case of concatenation.

Code (CSharp):

1.  
2.       // passes
3.       string concat1 = 1.ToString();
4.       UnityEngine.Debug.Log("`1.ToString()` passes. You WILL see this message.");
5.  
6.       // passes
7.       string concat2 = "" + 2.ToString();
8.       UnityEngine.Debug.Log("`\"\" + 2.ToString()` passes. You WILL see this message.");
9.      
10.       // passes
11.       string concat3 = "concat" + 3.ToString();
12.       UnityEngine.Debug.Log("`\"concat\" + 3.ToString()` passes. You WILL see this message.");
13.  
14.       // exception
15.       string concat4 = "concat" + 4;
16.       UnityEngine.Debug.Log("`\"concat\" + 4` causes an exception. You WONT see this message.");
17.      
18.       // wont build
19.       // error CS0656: Missing compiler required member 'System.String.Concat'
20. //      string concat5 = "" + 5;
21.  

In case 4 it seems the value type is being implicitly treated as a System.Object, thus causing a runtime error. In case 5, this problem is correctly identified at build time: *error CS0656: Missing compiler required member 'System.String.Concat'*; but this problem is not identified at build time in case 4. What I would expect is builds to always fail when value types are implicitly treated as a System.Object (ie, case 4 should not build).

 

I completely forgot to mention this is a WebAssmbly build

Regardless, I submitted the bug report; my case number is: `1228388`.