[TypeChecker] Require a coercion if result of protocol member access would loose information

Accessing members on the protocol could result in existential opening and subsequence
result erasure, which requires explicit coercion if there is any loss of generic requirements.

lib/Sema/ConstraintSystem.cpp

@@ -2296,11 +2296,30 @@ ConstraintSystem::getTypeOfMemberReference(
       baseObjTy->isExistentialType() && outerDC->getSelfProtocolDecl() &&
       // If there are no type variables, there were no references to 'Self'.
       type->hasTypeVariable()) {
+    auto getResultType = [](Type type) {
+      if (auto *funcTy = type->getAs<FunctionType>())
+        return funcTy->getResult();
+      return type;
+    };
+
+    auto nonErasedResultTy = getResultType(type);
+
     const auto selfGP = cast<GenericTypeParamType>(
         outerDC->getSelfInterfaceType()->getCanonicalType());
     auto openedTypeVar = replacements.lookup(selfGP);
     type = typeEraseOpenedExistentialReference(type, baseObjTy, openedTypeVar,
                                                TypePosition::Covariant);
+
+    if (!hasFixFor(locator) &&
+        AddExplicitExistentialCoercion::isRequired(
+            *this, nonErasedResultTy,
+            [&](TypeVariableType *typeVar) {
+              return openedTypeVar == typeVar ? baseObjTy : Optional<Type>();
+            },
+            locator)) {
+      recordFix(AddExplicitExistentialCoercion::create(
+          *this, getResultType(type), locator));
+    }
   }
 
   // Construct an idealized parameter type of the initializer associated