Partial class implementing identically named file interfaces with identically named members should not confuse them

[mkane91301]

Version Used: .NET 7

(Before you think this is a crazy scenario, the use case is generated code.)

Steps to Reproduce:
(Unfortunately, this can't be demonstrated in a gist, because it requires multiple files.)

1. In one file:

file interface IFoo
{
    IReadOnlyDictionary<int, string> Bar { get; }
}

internal partial class Foo : IFoo
{
    private readonly Dictionary<int, string> _bar = new() { { 1, "one" }, { 2, "two" } };
    IReadOnlyDictionary<int, string> IFoo.Bar => _bar;
}

2. In a second file:

file interface IFoo
{
    IReadOnlyDictionary<int, string> Bar { get; }
}

internal partial class Foo : IFoo
{
    IReadOnlyDictionary<int, string> IFoo.Bar => _bar;
}

3. After getting no complaints in Visual Studio, compile.

Diagnostic Id: CS0102

4. Change second file:

file interface IFoo
{
    IReadOnlyDictionary<int, string> Bar { get; }
}

internal partial class Foo : IFoo
{
}

(Foo needs to implement IFoo because, elsewhere in the file, an instance of Foo will be passed to a method taking IFoo.)

5. Visual Studio complains that Foo doesn't implement IFoo.

6. Compile.

Diagnostic Id: CS0535

Expected Behavior: The compiler recognizes that IFoo.Bar in the part of Foo in one file is different from IFoo.Bar in a different part because each IFoo is file scoped and is therefore a different type. Seeing these as different types, the compiler does not emit CS0102 and compiles the code.

Actual Behavior: The compiler confuses file scoped types with the same name and emits CS0102 and doesn't compile the code.

[RikkiGibson]

Thanks for reporting! Adding the full error message here, as we don't have the error codes memorized 😉:

Class2.cs(8,43,8,46): error CS0102: The type 'Foo' already contains a definition for 'IFoo.Bar'