[Mangler] Mangle generic typealiases in extensions of nested generics.

Author: DougGregor

lib/AST/ASTMangler.cpp

@@ -1045,6 +1045,12 @@ unsigned ASTMangler::appendBoundGenericArgs(DeclContext *dc,
   auto decl = dc->getInnermostDeclarationDeclContext();
   if (!decl) return 0;
 
+  // For an extension declaration, use the nominal type declaration instead.
+  // This is important when extending a nested type, because the generic
+  // parameters will line up with the (semantic) nesting of the nominal type.
+  if (auto ext = dyn_cast<ExtensionDecl>(decl))
+    decl = ext->getAsNominalTypeOrNominalTypeExtensionContext();
+
   // Handle the generic arguments of the parent.
   unsigned currentGenericParamIdx =
     appendBoundGenericArgs(decl->getDeclContext(), subs, isFirstArgList);

test/DebugInfo/DumpDeclFromMangledName.swift

@@ -51,6 +51,7 @@ typealias Patatino<T> = Foo<T>
 
 public struct Outer<T> {
   public struct Inner { }
+  public struct GenericInner<U> { }
 
   public typealias Foo<U> = Outer<U>.Inner
 
@@ -59,6 +60,14 @@ public struct Outer<T> {
   }
 }
 
+extension Outer.GenericInner {
+  public typealias Bar = Int
+
+  public func useBar() {
+    let bar: Bar = 7
+  }
+}
+
 func main() -> Int {
   var p : Patatino<Int> = Patatino(23);
   return 0

test/DebugInfo/Inputs/decl-reconstr-names.txt

@@ -2,3 +2,4 @@ $S12DeclReconstr8patatinoSiyF ---> func patatino() -> Int
 $S12DeclReconstr1SVACycfC ---> init()
 $S12DeclReconstr8PatatinoaySiGD ---> typealias Patatino<T> = Foo<T>
 $S12DeclReconstr5OuterV3Fooayx_SiGD ---> Can't resolve decl of $S12DeclReconstr5OuterV3Fooayx_SiGD
+$S12DeclReconstr5OuterV12GenericInnerV3Barayx_qd___GD ---> typealias Bar = Int