Build error that wasn't there before

Just to add to my previous response, if you're unsure which version of C# is being used by Azure, you can check the language version setting in the project properties. You can also use conditional compilation symbols to target different versions of C# based on the version being used by the target environment. For example, you can use the symbol "NETCOREAPP2_0" for .NET Core 2.0 and later, which supports C# 7.0 and later, and "NETCOREAPP1_0" for .NET Core 1.0 and earlier, which supports C# 6.0 and earlier. This allows you to write code that can work with different versions of C# based on the target environment.

Prior to C# 7.0, to check if a type variable is null, the == operator was used. However, starting from C# 7.0, the is operator can also be used to check if a type variable is null.

The suggestion is that the Azure platform might be using an earlier version of C# which does not support the use of the is operator to check for null. Therefore, to fix the compiler error, the is null expression in the code should be changed to == null. If the code is running on a version of C# earlier than 7.0, then the is null expression will not work, and the == null operator should be used instead to check if a type variable is null.
 

Starting from C# 7.0, the ability to compare a type with null using the is operator was introduced. This allows you to write expressions such as type is null to check if a type variable is null. However, prior to C# 7.0, you would have to compare a type variable to null explicitly using the == operator.

It's possible that the version of C# being used by Azure is earlier than version 7.0, which is causing the compiler error. To fix this, you can change the is null expression to == null in the first if statement:

if (type == null)
{
    throw new ArgumentNullException("type", "The supplied type must exist (ie cannot be null, which it is in this case).");
}

The error message suggests that the compiler is having trouble inferring the type of the type parameter in the method signature. It's possible that Azure is using a different version of the C# compiler, or that there is some other difference in the build environment that is causing the issue.

To fix the error, you can try explicitly specifying the type of the type parameter using the Type keyword, like this:

public static object GetValueOrFloor(this object potentiallyNullValue, Type type)
{
    if (type == null)
    {
        throw new ArgumentNullException(nameof(type), "The supplied type must exist (ie cannot be null, which it is in this case).");
    }
    else if (potentiallyNullValue != null && !type.Equals(potentiallyNullValue.GetType()))
    {
        throw new ArgumentException("The supplied type must match the type of object this method was called upon.");
    }

    if (type.Equals(typeof(DateTime)) || type.Equals(typeof(DateTime?)))
    {
        return potentiallyNullValue ?? DateTime.MinValue;
    }
    // ...
}

This should help the compiler correctly infer the type of the type parameter and resolve the CS1031 error.

It's also worth noting that the ArgumentNullException message on line 12 is using a string literal to reference the type parameter. It's generally better practice to use the nameof operator to reference the parameter name instead, to avoid potential errors if the parameter name is changed later on.