AST: Add TypeBase::eraseDynamicSelfType()

Author: slavapestov

include/swift/AST/Types.h

@@ -1343,6 +1343,10 @@ class alignas(1 << TypeAlignInBits) TypeBase
   /// argument labels removed.
   Type removeArgumentLabels(unsigned numArgumentLabels);
 
+  /// Replace DynamicSelfType anywhere it appears in covariant position with
+  /// its underlying Self type.
+  Type eraseDynamicSelfType();
+
   /// Replace DynamicSelfType anywhere it appears in covariant position with
   /// the given type.
   Type replaceDynamicSelfType(Type newSelfType);

lib/AST/Type.cpp

@@ -1342,19 +1342,34 @@ Type TypeBase::removeArgumentLabels(unsigned numArgumentLabels) {
   return FunctionType::get(unlabeledParams, result, fnType->getExtInfo());
 }
 
+Type TypeBase::eraseDynamicSelfType() {
+  if (!hasDynamicSelfType())
+    return Type(this);
+
+  return Type(this).transformWithPosition(
+      TypePosition::Covariant,
+      [&](TypeBase *t, TypePosition pos) -> std::optional<Type> {
+        if (isa<DynamicSelfType>(t) &&
+            pos == TypePosition::Covariant) {
+          return cast<DynamicSelfType>(t)->getSelfType();
+        }
+        return std::nullopt;
+      });
+}
+
 Type TypeBase::replaceDynamicSelfType(Type newSelfType) {
   if (!hasDynamicSelfType())
     return Type(this);
 
   return Type(this).transformWithPosition(
       TypePosition::Covariant,
       [&](TypeBase *t, TypePosition pos) -> std::optional<Type> {
-      if (isa<DynamicSelfType>(t) &&
-          pos == TypePosition::Covariant) {
-        return newSelfType;
-      }
-    return std::nullopt;
-  });
+        if (isa<DynamicSelfType>(t) &&
+            pos == TypePosition::Covariant) {
+          return newSelfType;
+        }
+        return std::nullopt;
+      });
 }
 
 Type TypeBase::withCovariantResultType() {