Bug 217681: compiler should not crash on public+private metadata

[jcouv]

This fixes this Watson-reported crash for update 3.

The crash occurs when accessibility flags on a method are set to 7 (which is all the flags, which means both public and private).

This problem existed on the VB side for a long time. On the C# side, before this change, those switch statements handled the default case with a debug assert and defaulting to "private".

On the C# side:

• native compiler treated those flags as "private" and reports errors:

consumer.cs(5,15): error CS1061: 'C' does not contain a definition for 'M' and no extension method 'M' accepting a first argument of type 'C' could be found (are you missing a using directive or an assembly reference?)
consumer.cs(6,40): error CS1061: 'C' does not contain a definition for 'F' and no extension method 'F' accepting a first argument of type 'C' could be found (are you missing a using directive or an assembly reference?)
consumer.cs(7,13): error CS0426: The type name 'C2' does not exist in the type 'C'

• compiler 1.2 compiler failed silently (no executable produced)
• future branch crashes
• after fix, errors are reported:

consumer.cs(5,15): error CS0122: 'C.M()' is inaccessible due to its protection level
consumer.cs(6,40): error CS0122: 'C.F' is inaccessible due to its protection level
consumer.cs(7,13): error CS0426: The type name 'C2' does not exist in the type 'C'

On the VB side:

• native compiler successfully compiles the method and field case (those only yield runtime error System.TypeLoadException: Invalid Field Access Flags) and reported an accessibility error on the nested type.
• compiler 1.2 generated errors:

error BC30390: 'C.Private Overloads Sub M()' is not accessible in this context because it is 'Private'.
error BC30389: 'C.F' is not accessible in this context because it is 'Private'.
error BC30389: 'C.C2' is not accessible in this context because it is 'Protected Friend'.
error BC30390: 'C2.Private Overloads Sub M2()' is not accessible in this context because it is 'Private'.

• future branch crashes
• after fix, the same errors as 1.2 are generated again. I will notify the compat council of this breaking with the native compiler.

[jcouv]

CC @dotnet/roslyn-compiler for review.

[jcouv]

@MattGertz Can you approve for update 3?
The compat council approved the small compat break.

[jcouv]

@dotnet/roslyn-compiler Please review for update 3.

[MattGertz]

Approved pending CR signoffs.

[AlekseyTs]

I assume () after C2 are not intentional. Never mind.

[AlekseyTs]

[AlekseyTs]

LGTM

[jaredpar]

👍