Ignored directives report errors in disabled regions

[jjonescz]

Version Used: main (7ebe9bf)

Steps to Reproduce:

#if false

System.Console.WriteLine("""
    #:sdk test
    """);

#endif

Expected Behavior: No error reported.

Actual Behavior:

// /Program.cs(4,6): error CS9298: '#:' directives can be only used in file-based programs ('-features:FileBasedProgram')
//     #:sdk test
Diagnostic(ErrorCode.ERR_PPIgnoredNeedsFileBasedProgram, ":").WithLocation(4, 6),
// /Program.cs(4,6): error CS9299: '#:' directives cannot be after '#if' directive
//     #:sdk test
Diagnostic(ErrorCode.ERR_PPIgnoredFollowsIf, ":").WithLocation(4, 6)

[RikkiGibson]

What is the representation of the content in the disabled region? I thought there would at least be a "string literal token" which contains the #:, and we would understand that it should not be lexed as a directive.

[jjonescz]

Only directives are parsed in disabled regions, string literals are not recognized.

See https://lab.razor.fyi/#4wrj4gooyk8vSszVSy4W8lDOTFNIS8wpTuXiCq4sLknN1XPOzyvOz0nVCy_KLEn1ycxL1VBSUuJSUFBQULYqTslWKEktLgFzlZSUNK25uJRT81Iy04TYDQ0MzQ0NTb14iivzShIrQooyyzITozg5jjW_vXM8VYAlgREA

CompilationUnit
└─EndOfFileToken ""
  └─LeadingTrivia
    ├─IfDirectiveTrivia "#if false"
    │ └─IfDirectiveTrivia
    │   ├─HashToken "#"
    │   ├─IfKeyword "if"
    │   │ └─TrailingTrivia
    │   │   └─WhitespaceTrivia " "
    │   ├─FalseLiteralExpression
    │   │ └─FalseKeyword "false"
    │   └─EndOfDirectiveToken ""
    │     └─TrailingTrivia
    │       └─EndOfLineTrivia "⏎"
    ├─DisabledTextTrivia "⏎System.Console.WriteLine("""⏎"
    ├─WhitespaceTrivia "    "
    ├─IgnoredDirectiveTrivia "#:sdk test"
    │ └─IgnoredDirectiveTrivia
    │   ├─HashToken "#"
    │   ├─ColonToken ":"
    │   ├─StringLiteralToken "sdk test"
    │   └─EndOfDirectiveToken ""
    │     └─TrailingTrivia
    │       └─EndOfLineTrivia "⏎"
    ├─DisabledTextTrivia "    """);⏎⏎"
    └─EndIfDirectiveTrivia "#endif"
      └─EndIfDirectiveTrivia
        ├─HashToken "#"
        ├─EndIfKeyword "endif"
        └─EndOfDirectiveToken ""

[jjonescz]

I guess this is a pre existing issue not really related to "ignored directives", same thing can be reproduced with any "unrecognized directive".

That seems unfortunate, because if you have compiler tests like

[Fact]
void Test()
{
    var source = """
        #test
        """;
}

they compile fine until you add something like #if NET9 around them - then they stop compiling when !NET9. (That's how I stumbled on this issue).

The C# standard says in https://github.com/dotnet/csharpstandard/blob/ba0673e920bf516bd0b99d21bdc1041f1d963ffa/standard/lexical-structure.md#655-conditional-compilation-directives:

Any remaining conditional sections are skipped and no tokens, except those for pre-processing directives, are generated from the source code. Therefore skipped source code, except pre-processing directives, may be lexically incorrect. Skipped pre-processing directives shall be lexically correct but are not otherwise processed. Within a conditional section that is being skipped any nested conditional sections (contained in nested #if...#endif constructs) are also skipped.

So the error might be expected since it says "Skipped pre-processing directives shall be lexically correct but are not otherwise processed."

[CyrusNajmabadi]

This is by design. Our preprocessor is not an actual pre processor. It's an in-situ lexer-controlling system. So once in a disabled block, there is no actual code, just scanning line for other directives. :)

[jjonescz]

Still as the linked PR shows, we could improve the UX for example at least for #: directives - by not reporting some errors in the disabled regions.

[CyrusNajmabadi]

Sure. It's up to you as to the semantics you want for that feature :)