Erase opaque types involving opened existential types to their upper bound.

Author: DougGregor

lib/AST/Type.cpp

@@ -507,6 +507,20 @@ Type TypeBase::typeEraseOpenedArchetypesWithRoot(
         return Type(ExistentialMetatypeType::get(erasedTy));
       }
 
+      // Opaque types whose substitutions involve this type parameter are
+      // erased to their upper bound.
+      if (auto opaque = dyn_cast<OpaqueTypeArchetypeType>(ty)) {
+        for (auto replacementType :
+                 opaque->getSubstitutions().getReplacementTypes()) {
+          if (replacementType->hasOpenedExistentialWithRoot(root)) {
+            Type interfaceType = opaque->getInterfaceType();
+            auto genericSig =
+                opaque->getDecl()->getOpaqueInterfaceGenericSignature();
+            return genericSig->getNonDependentUpperBounds(interfaceType);
+          }
+        }
+      }
+
       auto *const archetype = dyn_cast<OpenedArchetypeType>(ty);
       if (!archetype) {
         // Recurse.

lib/Sema/ConstraintSystem.cpp

@@ -1777,6 +1777,19 @@ static Type typeEraseCovariantExistentialSelfReferences(Type refTy,
 
   unsigned metatypeDepth = 0;
 
+  /// Check whether the given type has a reference to the generic parameter
+  /// that we are erasing.
+  auto hasErasedGenericParameter = [&](Type type) {
+    if (!type->hasTypeParameter())
+      return false;
+
+    return type.findIf([&](Type type) {
+      if (auto gp = type->getAs<GenericTypeParamType>())
+        return gp->getDepth() == 0;
+      return false;
+    });
+  };
+
   std::function<Type(Type, TypePosition)> transformFn;
   transformFn = [&](Type type, TypePosition initialPos) -> Type {
     return type.transformWithPosition(
@@ -1798,6 +1811,20 @@ static Type typeEraseCovariantExistentialSelfReferences(Type refTy,
         return Type(ExistentialMetatypeType::get(erasedTy));
       }
 
+      // Opaque types whose substitutions involve this type parameter are
+      // erased to their upper bound.
+      if (auto opaque = dyn_cast<OpaqueTypeArchetypeType>(type.getPointer())) {
+        for (auto replacementType :
+                 opaque->getSubstitutions().getReplacementTypes()) {
+          if (hasErasedGenericParameter(replacementType)) {
+            Type interfaceType = opaque->getInterfaceType();
+            auto genericSig =
+                opaque->getDecl()->getOpaqueInterfaceGenericSignature();
+            return genericSig->getNonDependentUpperBounds(interfaceType);
+          }
+        }
+      }
+
       if (!t->isTypeParameter()) {
         // Recurse.
         return None;

test/Constraints/opened_existentials.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-opened-existential-types -enable-parameterized-protocol-types -enable-experimental-opaque-parameters
+// RUN: %target-typecheck-verify-swift -enable-experimental-opened-existential-types -enable-parametrized-protocol-types -enable-experimental-opaque-parameters
 
 protocol Q { }
 
@@ -60,3 +60,20 @@ func testMultipleOpened(a: any P, b: any P & Q) {
   let r2 = a.combineThePs(b)
   let _: Int = r2  // expected-error{{cannot convert value of type '(Q, Q)?' to specified type 'Int'}}  
 }
+
+// --- With primary associated types and opaque parameter types
+protocol CollectionOf: Collection {
+  @_primaryAssociatedType associatedtype Element
+}
+
+extension Array: CollectionOf { }
+extension Set: CollectionOf { }
+
+func reverseIt<T>(_ c: some CollectionOf<T>) -> some CollectionOf<T> {
+  return c.reversed()
+}
+
+func useReverseIt(_ c: any CollectionOf) {
+  let c = reverseIt(c)
+  let _: Int = c // expected-error{{cannot convert value of type 'CollectionOf' to specified type 'Int'}}
+}