Merge pull request #42559 from xedin/se-0352-as-any-coercion

[TypeChecker] SE-0352: Require coercion if result type contains existential(s) that would loose generic requirements

lib/Sema/CSSimplify.cpp

@@ -1479,13 +1479,14 @@ shouldOpenExistentialCallArgument(
     return None;
 
   // An argument expression that explicitly coerces to an existential
-  // disables the implicit opening of the existential.
+  // disables the implicit opening of the existential unless it's
+  // wrapped in parens.
   if (argExpr) {
     if (auto argCast = dyn_cast<ExplicitCastExpr>(
             argExpr->getSemanticsProvidingExpr())) {
       if (auto typeRepr = argCast->getCastTypeRepr()) {
         if (auto toType = cs.getType(typeRepr)) {
-          if (toType->isAnyExistentialType())
+          if (!isa<ParenExpr>(argExpr) && toType->isAnyExistentialType())
             return None;
         }
       }
@@ -11463,6 +11464,18 @@ ConstraintSystem::simplifyApplicableFnConstraint(
             result2, opened.second->getExistentialType(), opened.first,
             TypePosition::Covariant);
       }
+
+      // If result type has any erased existential types it requires explicit
+      // `as` coercion.
+      if (AddExplicitExistentialCoercion::isRequired(
+              *this, func2->getResult(), openedExistentials, locator)) {
+        if (!shouldAttemptFixes())
+          return SolutionKind::Error;
+
+        if (recordFix(AddExplicitExistentialCoercion::create(
+                *this, result2, getConstraintLocator(locator))))
+          return SolutionKind::Error;
+      }
     }
 
     // The result types are equivalent.
@@ -13121,6 +13134,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::DropAsyncAttribute:
   case FixKind::AllowSwiftToCPointerConversion:
   case FixKind::AllowTupleLabelMismatch:
+  case FixKind::AddExplicitExistentialCoercion:
     llvm_unreachable("handled elsewhere");
   }