AST: add TypeBase::canBeExistential()

Checks if the type is an existential or an archetype which may be an existential.

Author: eeckstein

include/swift/AST/Types.h

@@ -752,6 +752,10 @@ class alignas(1 << TypeAlignInBits) TypeBase
     return getRecursiveProperties().hasOpenedExistential();
   }
 
+  /// True if this type is an existential or an archetype which may be an
+  /// existential.
+  bool canBeExistential();
+
   /// Determine whether the type involves an opened element archetype.
   bool hasElementArchetype() const {
     return getRecursiveProperties().hasElementArchetype();

lib/AST/Type.cpp

@@ -616,6 +616,29 @@ bool TypeBase::isTypeVariableOrMember() {
   return getDependentMemberRoot()->is<TypeVariableType>();
 }
 
+bool TypeBase::canBeExistential() {
+  if (isAnyExistentialType())
+    return true;
+
+  Type ty(this);
+  // Unwrap (potentially multiple levels of) metatypes.
+  while (auto *mt = ty->getAs<MetatypeType>())
+    ty = mt->getInstanceType();
+
+  if (auto *archeTy = ty->getAs<ArchetypeType>()) {
+    // Only if all conformances are self-conforming protocols, the archetype
+    // may be an existential.
+    for (auto *proto : archeTy->getConformsTo()) {
+      if (!proto->existentialConformsToSelf())
+        return false;
+    }
+    // If there are no requirements on the archetype at all (`getConformsTo`
+    // is empty), the archetype can still be `Any` and we have to return true.
+    return true;
+  }
+  return false;
+}
+
 bool TypeBase::isTypeParameter() {
   return getDependentMemberRoot()->is<GenericTypeParamType>();
 }