[PrintAsObjC] Handle generic typealiases (#18941)

(either explicitly generic ones, or those embedded in generic contexts)

Previously we tried to look at the typealias decl's underlying type,
but that might have generic parameters in it. Oops. Use the
NameAliasType's desugared type instead.

Also, drop half-baked support for `@objc typealias`, which isn't
supported. (It's not an unreasonable feature, but the bits that were
there weren't implemented correctly.)

rdar://problem/43347303

Author: jrose-apple

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -1523,12 +1523,8 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
 
   bool printImportedAlias(const TypeAliasDecl *alias,
                           Optional<OptionalTypeKind> optionalKind) {
-    if (!alias->hasClangNode()) {
-      if (!alias->isObjC()) return false;
-
-      os << alias->getName();
-      return true;
-    }
+    if (!alias->hasClangNode())
+      return false;
 
     if (auto *clangTypeDecl =
           dyn_cast<clang::TypeDecl>(alias->getClangDecl())) {
@@ -1560,7 +1556,7 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
     if (printImportedAlias(alias, optionalKind))
       return;
 
-    visitPart(alias->getUnderlyingTypeLoc().getType(), optionalKind);
+    visitPart(aliasTy->getSinglyDesugaredType(), optionalKind);
   }
 
   void maybePrintTagKeyword(const TypeDecl *NTD) {
@@ -1993,7 +1989,10 @@ class ReferencedTypeFinder : public TypeVisitor<ReferencedTypeFinder> {
   }
 
   void visitNameAliasType(NameAliasType *aliasTy) {
-    Callback(*this, aliasTy->getDecl());
+    if (aliasTy->getDecl()->hasClangNode())
+      Callback(*this, aliasTy->getDecl());
+    else
+      visit(aliasTy->getSinglyDesugaredType());
   }
 
   void visitParenType(ParenType *parenTy) {
@@ -2312,9 +2311,8 @@ class ModuleWriter {
         } else if (auto PD = dyn_cast<ProtocolDecl>(TD)) {
           forwardDeclare(PD);
         } else if (auto TAD = dyn_cast<TypeAliasDecl>(TD)) {
-          (void)addImport(TD);
-          // Just in case, make sure the underlying type is visible too.
-          finder.visit(TAD->getUnderlyingTypeLoc().getType());
+          if (TAD->hasClangNode())
+            (void)addImport(TD);
         } else if (addImport(TD)) {
           return;
         } else if (auto ED = dyn_cast<EnumDecl>(TD)) {

test/PrintAsObjC/typealias.swift

@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module -o %t/typealias.swiftmodule %s -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend -parse-as-library %t/typealias.swiftmodule -typecheck -emit-objc-header-path %t/typealias.h -disable-objc-attr-requires-foundation-module
+// RUN: %FileCheck %s < %t/typealias.h
+// RUN: %check-in-clang %t/typealias.h
+
+// RUN: %target-swift-frontend -typecheck %s -emit-objc-header-path %t/typealias_wmo.h -disable-objc-attr-requires-foundation-module
+// RUN: %FileCheck %s < %t/typealias_wmo.h
+// RUN: %check-in-clang %t/typealias_wmo.h
+
+// REQUIRES: objc_interop
+
+public typealias ActuallyGeneric<T> = T
+
+public struct GenericHelper<T> {
+  public typealias TheType = T
+}
+public typealias PassesThroughGeneric = GenericHelper<Int>.TheType
+
+// CHECK-LABEL: @interface X
+@objc public class X {
+  // CHECK: @property (nonatomic) NSInteger actuallyGeneric;
+  @objc public var actuallyGeneric: ActuallyGeneric<Int> = 0
+  // CHECK: @property (nonatomic) NSInteger passesThroughGeneric;
+  @objc public var passesThroughGeneric: PassesThroughGeneric = 0
+} // CHECK: @end