Empty Lexical Context When Expanding Expression Macro as Argument's Default Value

[filip-sakel]

Description

I'm trying to use a freestanding expression macro as the default value of a function argument, as outlined in SE-0422. This macro accesses the lexical context, which is unfortunately empty when the macro is expanded as the default value to an argument. I can only get the lexical context when explicitly passing in the macro to the argument.

Reproduction

Client.swift

@freestanding(expression)
public macro _environmentValuesContext() = 
    #externalMacro(module: "Macro", type: "EnvironmentValuesMacro")

struct MyStruct {
    func useContext(_ context: Void = #_environmentValuesContext) {}

    func run() {
        // Nothing when expanded as the default value for the argument
        useContext() // Lexical context: [] (from macro '_environmentValuesContext')

        // Correct context when passing macro explicitly
        useContext(#_environmentValuesContext) // Lexical context: [FunctionDeclSyntax ...
    }
}

Macro.swift

import SwiftSyntax
import SwiftSyntaxMacros
import SwiftCompilerPlugin
import SwiftDiagnostics

@main
struct Macro: CompilerPlugin {
    let providingMacros: [Macro.Type] = [
        EnvironmentValuesMacro.self,
    ]
}

public struct EnvironmentValuesMacro: ExpressionMacro {
    public static func expansion(
        of node: some FreestandingMacroExpansionSyntax, 
        in context: some MacroExpansionContext
    ) throws -> ExprSyntax {
        context.diagnose(Diagnostic(
            node: node, 
            message: SimpleDiagnosticMessage(
                message: "Lexical context: \(context.lexicalContext)", 
                severity: .error
            )
        ))
        return "()"
    }
}

Expected behavior

I would expect that the lexical context is passed to the macro even when the macro is expanded as the argument's default value, i.e. useContext() and useContext(#_environmentValuesContext) should yield the same result.

Environment

Swift version 6.2-dev (LLVM f3274059e0a182e, Swift 442f25d)
Target: aarch64-unknown-linux-gnu
Build config: +assertions

Additional information

No response

[ValentinWalter]

I recently ran into this too. Hoping it's indeed a bug and not intentional. My use case is a #sourceLocation macro that reads the lexical context to provide a rich description of the source location to a logger.