[SE-0407] Add missingConformancesTo argument to MemberMacro expansion operation

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -238,19 +238,12 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
         throw MacroExpansionError.declarationNotDeclGroup
       }
 
-      // Local function to expand a member macro once we've opened up
-      // the existential.
-      func expandMemberMacro(
-        _ node: some DeclGroupSyntax
-      ) throws -> [DeclSyntax] {
-        return try attachedMacro.expansion(
-          of: attributeNode,
-          providingMembersOf: node,
-          in: context
-        )
-      }
-
-      let members = try _openExistential(declGroup, do: expandMemberMacro)
+      let members = try attachedMacro.expansion(
+        of: attributeNode,
+        providingMembersOf: declGroup,
+        conformingTo: conformanceList?.map(\.typeName) ?? [],
+        in: context
+      )
 
       // Form a buffer of member declarations to return to the caller.
       return members.map { $0.formattedExpansion(definition.formatMode) }

Sources/SwiftSyntaxMacros/MacroProtocols/MemberMacro.swift

@@ -23,9 +23,54 @@ public protocol MemberMacro: AttachedMacro {
   ///
   /// - Returns: the set of member declarations introduced by this macro, which
   /// are nested inside the `attachedTo` declaration.
+  @available(*, deprecated, message: "Use expansion(of:providingMembersOf:conformingTo:in:")
   static func expansion(
     of node: AttributeSyntax,
     providingMembersOf declaration: some DeclGroupSyntax,
     in context: some MacroExpansionContext
   ) throws -> [DeclSyntax]
+
+  /// Expand an attached declaration macro to produce a set of members.
+  ///
+  /// - Parameters:
+  ///   - node: The custom attribute describing the attached macro.
+  ///   - declaration: The declaration the macro attribute is attached to.
+  ///   - conformingTo: The set of protocols that were declared
+  ///     in the set of conformances for the macro and to which the declaration
+  ///     does not explicitly conform. The member macro itself cannot declare
+  ///     conformances to these protocols (only an extension macro can do that),
+  ///     but can provide supporting declarations, such as a required
+  ///     initializer or stored property, that cannot be written in an
+  ///     extension.
+  ///   - context: The context in which to perform the macro expansion.
+  ///
+  /// - Returns: the set of member declarations introduced by this macro, which
+  /// are nested inside the `attachedTo` declaration.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax]
+}
+
+public extension MemberMacro {
+  /// Default implementation supplies no conformances.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return try expansion(of: node, providingMembersOf: declaration, conformingTo: [], in: context)
+  }
+
+  /// Default implementation that ignores the unhandled conformances.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return try expansion(of: node, providingMembersOf: declaration, in: context)
+  }
 }