Add behind experimental feature flag

Author: Azoy

CodeGeneration/Sources/SyntaxSupport/ExperimentalFeatures.swift

@@ -23,6 +23,7 @@ public enum ExperimentalFeature: String, CaseIterable {
   case oldOwnershipOperatorSpellings
   case defaultIsolationPerFile
   case moduleSelector
+  case defaultGenerics
 
   /// The name of the feature as it is written in the compiler's `Features.def` file.
   public var featureName: String {
@@ -47,6 +48,8 @@ public enum ExperimentalFeature: String, CaseIterable {
       return "DefaultIsolationPerFile"
     case .moduleSelector:
       return "ModuleSelector"
+    case .defaultGenerics:
+      return "DefaultGenerics"
     }
   }
 
@@ -73,6 +76,8 @@ public enum ExperimentalFeature: String, CaseIterable {
       return "set default actor isolation for a file"
     case .moduleSelector:
       return "Module selector syntax (`ModName::identifier`)"
+    case .defaultGenerics:
+      return "default generics"
     }
   }
 

CodeGeneration/Sources/SyntaxSupport/GenericNodes.swift

@@ -126,15 +126,11 @@ public let GENERIC_NODES: [Node] = [
         isOptional: true
       ),
       Child(
-        name: "equal",
-        kind: .token(choices: [.token(.equal)]),
-        isOptional: true
-      ),
-      Child(
-        name: "defaultType",
-        kind: .node(kind: .type),
+        name: "initializer",
+        kind: .node(kind: .typeInitializerClause),
+        experimentalFeature: .defaultGenerics,
         nameForDiagnostics: "default type",
-        isOptional: true
+        isOptional: true,
       ),
       Child(
         name: "trailingComma",

Sources/SwiftParser/Declarations.swift

@@ -676,30 +676,19 @@ extension Parser {
           inherited = nil
         }
 
-        // Parse the '=' followed by a type.
-        let equal = self.consume(if: .equal)
-        let unexpectedBeforeDefault: RawUnexpectedNodesSyntax?
-        let defaultType: RawTypeSyntax?
-        if equal != nil {
-          if self.at(.identifier, .keyword(.protocol), .keyword(.Any)) || self.atContextualPunctuator("~") {
-            unexpectedBeforeDefault = nil
-            defaultType = self.parseType()
-          } else if let classKeyword = self.consume(if: .keyword(.class)) {
-            unexpectedBeforeDefault = RawUnexpectedNodesSyntax([classKeyword], arena: self.arena)
-            defaultType = RawTypeSyntax(
-              RawIdentifierTypeSyntax(
-                moduleSelector: nil,
-                name: missingToken(.identifier, text: "AnyObject"),
-                genericArgumentClause: nil,
-                arena: self.arena
-              )
-            )
-          } else {
-            unexpectedBeforeDefault = nil
-            defaultType = RawTypeSyntax(RawMissingTypeSyntax(arena: self.arena))
-          }
+        // Parse the default type, if any. We only have defaults of regular
+        // type parameters, not parameter packs or value generics yet.
+        let defaultType: RawTypeInitializerClauseSyntax?
+        if self.experimentalFeatures.contains(.defaultGenerics),
+           specifier == nil,
+           let equal = self.consume(if: .equal) {
+          let type = self.parseType()
+          defaultType = RawTypeInitializerClauseSyntax(
+            equal: equal,
+            value: type,
+            arena: self.arena
+          )
         } else {
-          unexpectedBeforeDefault = nil
           defaultType = nil
         }
 
@@ -714,9 +703,7 @@ extension Parser {
             colon: colon,
             unexpectedBeforeInherited,
             inheritedType: inherited,
-            equal: equal,
-            unexpectedBeforeDefault,
-            defaultType: defaultType,
+            initializer: defaultType,
             trailingComma: keepGoing,
             arena: self.arena
           )