Add a group around member types used in return clauses.

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -1049,6 +1049,13 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
 
   override func visit(_ node: ReturnClauseSyntax) -> SyntaxVisitorContinueKind {
     after(node.arrow, tokens: .space)
+
+    // Member type identifier is used when the return type is a member of another type. Add a group
+    // here so that the base, dot, and member type are kept together when they fit.
+    if node.returnType.is(MemberTypeIdentifierSyntax.self) {
+      before(node.returnType.firstToken, tokens: .open)
+      after(node.returnType.lastToken, tokens: .close)
+    }
     return .visitChildren
   }
 

Tests/SwiftFormatPrettyPrintTests/FunctionDeclTests.swift

@@ -90,7 +90,6 @@ final class FunctionDeclTests: PrettyPrintTestCase {
   }
 
   func testFunctionDeclReturns() {
-    // TODO: The tuple return case needs a lot of work.
     let input =
       """
       func myFun(var1: Int, var2: Double) -> Double {
@@ -104,6 +103,12 @@ final class FunctionDeclTests: PrettyPrintTestCase {
       func tupleFunc() throws -> (one: Int, two: Double, three: Bool, four: String) {
         return (one: 1, two: 2.0, three: true, four: "four")
       }
+      func memberTypeThrowingFunc() throws -> SomeBaseType<GenericArg1, GenericArg2, GenericArg3>.SomeInnerType {
+      }
+      func memberTypeReallyLongNameFunc() -> Type.InnerMember {
+      }
+      func tupleMembersFunc() -> (Type.Inner, Type2.Inner2) {
+      }
       """
 
     let expected =
@@ -125,6 +130,20 @@ final class FunctionDeclTests: PrettyPrintTestCase {
           one: 1, two: 2.0, three: true, four: "four"
         )
       }
+      func memberTypeThrowingFunc() throws
+        -> SomeBaseType<
+          GenericArg1, GenericArg2, GenericArg3
+        >.SomeInnerType
+      {
+      }
+      func memberTypeReallyLongNameFunc()
+        -> Type.InnerMember
+      {
+      }
+      func tupleMembersFunc() -> (
+        Type.Inner, Type2.Inner2
+      ) {
+      }
 
       """