[Mangling] Don't mangle generic typealiases.

Mangling of generic typealiases for debug info is causing a long tail of
bugs. Disable it on the Swift 4.2 branch; we'll fall back to the
underlying type.

Fixes rdar://problem/41444286.

Author: DougGregor

lib/AST/ASTMangler.cpp

@@ -739,13 +739,16 @@ void ASTMangler::appendType(Type type) {
       auto aliasTy = cast<NameAliasType>(tybase);
 
       // It's not possible to mangle the context of the builtin module.
-      // FIXME: We also cannot yet mangle references to typealiases that
-      // involve generics.
       TypeAliasDecl *decl = aliasTy->getDecl();
       if (decl->getModuleContext() == decl->getASTContext().TheBuiltinModule) {
         return appendType(aliasTy->getSinglyDesugaredType());
       }
 
+      // FIXME: We also cannot yet mangle references to typealiases that
+      // involve generics.
+      if (decl->getGenericSignature())
+        return appendSugaredType<NameAliasType>(type);
+
       // For the DWARF output we want to mangle the type alias + context,
       // unless the type alias references a builtin type.
       return appendAnyGenericType(decl);

test/ClangImporter/objc_ir.swift

@@ -363,7 +363,5 @@ func testBlocksWithGenerics(hba: HasBlockArray) -> Any {
 // CHECK: ![[SWIFT_NAME_ALIAS_TYPE]] = !DIDerivedType(tag: DW_TAG_typedef, name: "$SSo14SwiftNameAliasaD", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, baseType: !{{[0-9]+}})
 
 // CHECK: ![[SWIFT_GENERIC_NAME_ALIAS_VAR]] = !DILocalVariable(name: "generic_obj", arg: 1, scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: ![[SWIFT_GENERIC_NAME_ALIAS_TYPE:[0-9]+]])
-// CHECK: ![[SWIFT_GENERIC_NAME_ALIAS_TYPE]] = !DIDerivedType(tag: DW_TAG_typedef, name: "$SSo21SwiftGenericNameAliasaySo8NSNumberCGD", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, baseType: !{{[0-9]+}})
 
 // CHECK: ![[SWIFT_CONSTR_GENERIC_NAME_ALIAS_VAR]] = !DILocalVariable(name: "constr_generic_obj", arg: 1, scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: ![[SWIFT_CONSTR_GENERIC_NAME_ALIAS_TYPE:[0-9]+]])
-// CHECK: ![[SWIFT_CONSTR_GENERIC_NAME_ALIAS_TYPE]] = !DIDerivedType(tag: DW_TAG_typedef, name: "$SSo27SwiftConstrGenericNameAliasaySo8NSNumberCGD", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, baseType: !{{[0-9]+}})

test/DebugInfo/DumpDeclFromMangledName.swift

@@ -39,3 +39,56 @@ func patatino() -> Int {
 }
 
 patatino()
+
+class Foo<T> {
+  var x : T
+  init(_ x : T) {
+    self.x = x
+  }
+}
+
+typealias Patatino<T> = Foo<T>
+
+public struct Outer<T> {
+  public struct Inner { }
+  public struct GenericInner<U> { }
+
+  public typealias Foo<U> = Outer<U>.Inner
+
+  public func blah() {
+    let foo: Foo<Int> = Outer<Int>.Inner()
+  }
+}
+
+extension Outer.GenericInner {
+  public typealias Bar = Int
+
+  public func useBar() {
+    let bar: Bar = 7
+  }
+}
+
+protocol P {
+  associatedtype A
+}
+
+protocol Q {
+  associatedtype B: P
+  typealias ProtocolTypeAliasThing = B.A
+}
+
+struct ConformsToP: P {
+  typealias A = Int
+}
+
+struct ConformsToQ: Q {
+  typealias B = ConformsToP
+}
+
+struct Blah {
+  typealias SomeQ = ConformsToQ
+
+  func foo() {
+    let bar: SomeQ.ProtocolTypeAliasThing? = nil
+  }
+}