Pattern matching fails on types with implicit conversion to string

[AArnott]

I noticed today that pattern matching switch statements do not compile if the patterned type is implicitly convertible to string. Is this a C# language spec issue, or a bug in the roslyn compiler?

Full repro

class M {
    public static implicit operator string(M value) => "";
    
    public bool P {get;set;}
    
    void B(M m) {
        switch (m) {
            case M1 { P: true } : break;
            case M2 : break;
        }
    }
}

class M1 : M {}
class M2 : M {}

error CS8121: An expression of type 'string' cannot be handled by a pattern of type 'M1'.
error CS8121: An expression of type 'string' cannot be handled by a pattern of type 'M2'.

[HaloFour]

This feels like it might be legacy switch baggage, in that it used to allow expressions to be used in switch statements that were implicitly converted to string before pattern matching existed?