[GSB] Merge layout constraints when merging equivalence classes.

lib/AST/GenericSignatureBuilder.cpp

@@ -4106,8 +4106,6 @@ static ConstraintResult visitInherited(
   ConstraintResult result = ConstraintResult::Resolved;
   std::function<void(Type, const TypeRepr *)> visitInherited;
 
-  // FIXME: Should this whole thing use getExistentialLayout() instead?
-
   visitInherited = [&](Type inheritedType, const TypeRepr *typeRepr) {
     // Decompose explicitly-written protocol compositions.
     if (auto composition = dyn_cast_or_null<CompositionTypeRepr>(typeRepr)) {
@@ -4938,6 +4936,22 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenTypeParameters(
   // Make T1 the representative of T2, merging the equivalence classes.
   T2->representativeOrEquivClass = T1;
 
+  // Layout requirements.
+  if (equivClass2 && equivClass2->layout) {
+    if (!equivClass->layout) {
+      equivClass->layout = equivClass2->layout;
+      equivClass->layoutConstraints = std::move(equivClass2->layoutConstraints);
+    } else {
+      const RequirementSource *source2
+        = equivClass2->layoutConstraints.front().source;
+      addLayoutRequirementDirect(T1, equivClass2->layout, source2);
+      equivClass->layoutConstraints.insert(
+                                   equivClass->layoutConstraints.end(),
+                                   equivClass2->layoutConstraints.begin() + 1,
+                                   equivClass2->layoutConstraints.end());
+    }
+  }
+
   // Superclass requirements.
   if (equivClass2 && equivClass2->superclass) {
     const RequirementSource *source2;

test/Generics/class_constraint.swift

@@ -25,3 +25,23 @@ struct Z<U> {
   let bad3: X<U> // expected-error{{'X' requires that 'U' be a class type}}
 }
 
+// SR-7168: layout constraints weren't getting merged.
+protocol P1 {
+  associatedtype A
+  var a: A { get }
+}
+
+protocol P2 {
+  associatedtype B: P1
+  var b: B { get }
+}
+
+func requiresAnyObject<T: AnyObject>(_: T) { }
+
+func anyObjectConstraint<T: P2, U: P2>(_ t: T, _ u: U)
+where T.B.A: AnyObject, U.B: AnyObject, T.B == T.B.A, U.B.A == U.B {
+  requiresAnyObject(t.b)
+  requiresAnyObject(u.b)
+  requiresAnyObject(t.b.a)
+  requiresAnyObject(u.b.a)
+}