Add support for generic parameters to have default types

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

@@ -125,6 +125,13 @@ public let GENERIC_NODES: [Node] = [
         nameForDiagnostics: "inherited type",
         isOptional: true
       ),
+      Child(
+        name: "initializer",
+        kind: .node(kind: .typeInitializerClause),
+        experimentalFeature: .defaultGenerics,
+        nameForDiagnostics: "default type",
+        isOptional: true,
+      ),
       Child(
         name: "trailingComma",
         kind: .token(choices: [.token(.comma)]),

Release Notes/603.md

@@ -0,0 +1,24 @@
+# Swift Syntax 603 Release Notes
+
+## New APIs
+
+- `GenericParameterSyntax` now has a new `initializer` property.
+  - Description: Generic parameters can now optionally be passed a default type. The `initializer` is a `TypeInitializerSyntax` that captures both the `=` and the parsed `TypeSyntax`.
+  - Pull Request: https://github.com/swiftlang/swift-syntax/pull/3152
+
+## API Behavior Changes
+
+## Deprecations
+
+## API-Incompatible Changes
+
+## Template
+
+- *Affected API or two word description*
+  - Description: *A 1-2 sentence description of the new/modified API*
+  - Issue: *If an issue exists for this change, a link to the issue*
+  - Pull Request: *Link to the pull request(s) that introduces this change*
+  - Migration steps: Steps that adopters of swift-syntax should take to move to the new API (required for deprecations and API-incompatible changes).
+  - Notes: *In case of deprecations or API-incompatible changes, the reason why this change was made and the suggested alternative*
+
+*Insert entries in chronological order, with newer entries at the bottom*

Sources/SwiftParser/Declarations.swift

@@ -675,6 +675,24 @@ extension Parser {
           unexpectedBeforeInherited = nil
           inherited = nil
         }
+
+        // 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 {
+          defaultType = nil
+        }
+
         keepGoing = self.consume(if: .comma)
         elements.append(
           RawGenericParameterSyntax(
@@ -686,6 +704,7 @@ extension Parser {
             colon: colon,
             unexpectedBeforeInherited,
             inheritedType: inherited,
+            initializer: defaultType,
             trailingComma: keepGoing,
             arena: self.arena
           )