[CSFix] Bound Generic Class Types require explicit coercion due to loss of generic parameters

Author: angela-laar

lib/Sema/CSFix.cpp

@@ -2506,6 +2506,14 @@ bool AddExplicitExistentialCoercion::isRequired(
           RequiresCoercion = true;
           return Action::Stop;
         }
+        
+        if (auto *const nominal = erasedMemberTy->getAs<NominalOrBoundGenericNominalType>()) {
+          // Bound Generic Nominals 
+          if (!hasConstrainedAssociatedTypes(member, *existentialType)) {
+            RequiresCoercion = true;
+            return Action::Stop;
+          }
+        }
 
         return Action::SkipChildren;
       }
@@ -2554,6 +2562,7 @@ bool AddExplicitExistentialCoercion::isRequired(
         case RequirementKind::Layout: {
           if (isAnchoredOn(req.getFirstType(), member->getAssocType()))
             return true;
+          
           break;
         }
 

test/Constraints/opened_existentials.swift

@@ -305,3 +305,42 @@ func testExplicitCoercionRequirement(v: any B, otherV: any B & D) {
   getP((getC(v) as any P))   // Ok - parens avoid opening suppression
   getP((v.getC() as any P))  // Ok - parens avoid opening suppression
 }
+
+// Generic Class Types
+class C1 {}
+class C2<T>: C1 {}
+
+// Protocols With Associated Types
+protocol Q {
+  associatedtype A
+  associatedtype B: C2<A>
+
+  func returnAssocTypeB() -> B
+}
+
+func testAssocReturn(p: any Q) {
+  let _ = p.returnAssocTypeB() // expected-error {{inferred result type 'C2<(any U<String>).A>' requires explicit coercion due to loss of generic requirements}} {{14-14=as as C2<(any Q).A>}}
+}
+
+// Protocols With Primary Associated Types
+protocol U<A> {
+  associatedtype A
+  associatedtype B: C2<A>
+
+  func returnAssocTypeB() -> B
+  func returnPrimaryAssocTypeA() -> A
+  func returnAssocTypeCollection() -> any Collection<A>
+}
+
+func testPrimaryAssocReturn(p: any U<String>) {
+  let _ = p.returnAssocTypeB()
+}
+
+func testPrimaryAssocReturn(p: any U<Int>) {
+  let _ = p.returnPrimaryAssocTypeA() // what would the output even be here?
+}
+
+func testPrimaryAssocCollection(p: any U<Float>) -> any Collection {
+  let x = p.returnAssocTypeCollection()
+  return x
+}