[AST] Add TypeBase::lookThroughSingleOptionalType

Author: hamishknight

include/swift/AST/Types.h

@@ -1067,6 +1067,9 @@ class alignas(1 << TypeAlignInBits) TypeBase {
   // otherwise, return the null type.
   Type getSwiftNewtypeUnderlyingType();
 
+  /// Return the type T after looking through at most one optional type.
+  Type lookThroughSingleOptionalType();
+
   /// Return the type T after looking through all of the optional
   /// types.
   Type lookThroughAllOptionalTypes();

lib/AST/Type.cpp

@@ -670,6 +670,13 @@ Type TypeBase::getAnyBufferPointerElementType(BufferPointerTypeKind &BPTK) {
   return Type();
 }
 
+Type TypeBase::lookThroughSingleOptionalType() {
+  Type type(this);
+  if (auto objType = type->getOptionalObjectType())
+    type = objType;
+  return type;
+}
+
 Type TypeBase::lookThroughAllOptionalTypes() {
   Type type(this);
   while (auto objType = type->getOptionalObjectType())

lib/Sema/CSSimplify.cpp

@@ -7241,10 +7241,8 @@ ConstraintSystem::simplifyDynamicCallableApplicableFnConstraint(
 }
 
 static Type getBaseTypeForPointer(ConstraintSystem &cs, TypeBase *type) {
-  if (Type unwrapped = type->getOptionalObjectType())
-    type = unwrapped.getPointer();
-
-  auto pointeeTy = type->getAnyPointerElementType();
+  auto pointeeTy = type->lookThroughSingleOptionalType()
+                       ->getAnyPointerElementType();
   assert(pointeeTy);
   return pointeeTy;
 }