AST: Break circularity in PackArchetypeType::getReducedShape()

This can't happen yet, but with an upcoming change, we hit an infinite
recursion here with PackExpansionType::get() -> getReducedShape()
-> mapTypeIntoContext() -> PackExpansionType::get().

Author: slavapestov

include/swift/AST/Types.h

@@ -6414,7 +6414,7 @@ class PackArchetypeType final
       LayoutConstraint Layout);
 
   // Returns the reduced shape type for this pack archetype.
-  CanType getReducedShape() const;
+  CanType getReducedShape();
 
   static bool classof(const TypeBase *T) {
     return T->getKind() == TypeKind::PackArchetype;

lib/AST/Type.cpp

@@ -3734,9 +3734,18 @@ PackArchetypeType::get(const ASTContext &Ctx,
       {ShapeType}));
 }
 
-CanType PackArchetypeType::getReducedShape() const {
+CanType PackArchetypeType::getReducedShape() {
+  // mapTypeIntoContext() also calls getReducedShape() via
+  // PackExpansionType::get(), so avoid that by short-circuiting
+  // the case where the pack archetype represents its own
+  // shape class.
   auto shapeType = getTrailingObjects<PackShape>()->shapeType;
-  return getGenericEnvironment()->mapTypeIntoContext(shapeType)->getCanonicalType();
+  if (shapeType->isEqual(getInterfaceType()))
+    return CanType(this);
+
+  return getGenericEnvironment()
+      ->mapTypeIntoContext(shapeType)
+      ->getCanonicalType();
 }
 
 ElementArchetypeType::ElementArchetypeType(