Don't print 'convenience' on protocol extension initializers (swiftlang#18777)

We model them that way in the compiler, but that's not part of the
user-level language.

rdar://problem/32067077

Author: jrose-apple

lib/AST/ASTPrinter.cpp

@@ -2707,10 +2707,25 @@ void PrintAST::visitConstructorDecl(ConstructorDecl *decl) {
   if ((decl->getInitKind() == CtorInitializerKind::Convenience ||
        decl->getInitKind() == CtorInitializerKind::ConvenienceFactory) &&
       !decl->getAttrs().hasAttribute<ConvenienceAttr>()) {
-    Printer.printKeyword("convenience");
-    Printer << " ";
+    // Protocol extension initializers are modeled as convenience initializers,
+    // but they're not written that way in source. Check if we're actually
+    // printing onto a class.
+    bool isClassContext;
+    if (CurrentType) {
+      isClassContext = CurrentType->getClassOrBoundGenericClass() != nullptr;
+    } else {
+      const DeclContext *dc = decl->getDeclContext();
+      isClassContext = dc->getSelfClassDecl() != nullptr;
+    }
+    if (isClassContext) {
+      Printer.printKeyword("convenience");
+      Printer << " ";
+    } else {
+      assert(decl->getDeclContext()->getExtendedProtocolDecl() &&
+             "unexpected convenience initializer");
+    }
   } else if (decl->getInitKind() == CtorInitializerKind::Factory) {
-      Printer << "/*not inherited*/ ";
+    Printer << "/*not inherited*/ ";
   }
 
   printContextIfNeeded(decl);

test/SourceKit/DocSupport/Inputs/cake1.swift

@@ -29,3 +29,19 @@ public extension P2 where Self : P3 {
 public extension Dictionary.Keys {
   public func foo() {}
 }
+
+public protocol InitProto {
+  init(x: Int)
+}
+extension InitProto {
+  // This initializer is marked as 'CtorInitializerKind::Convenience'.
+  public init() { self = Self(x: 0) }
+}
+
+public struct InitStructImpl : InitProto {
+  public init(x: Int) {}
+}
+
+public class InitClassImpl : InitProto {
+  public required init(x: Int) {}
+}