Clarify parsing ambiguity FIXME comments.

Author: dan-zheng

lib/Parse/ParseDecl.cpp

@@ -1071,10 +1071,20 @@ static bool parseBaseTypeForQualifiedDeclName(Parser &P, TypeRepr *&baseType) {
   // leading period unparsed to avoid syntax verification errors.
   assert(P.startsWithSymbol(P.Tok, '.') && "false");
 
-  // Check if this is a reference to an accessor in a computed property.
-  // FIXME: There is an ambiguity here because instead of a computed
-  // property with an accessor, this could be a type with a function
-  // name like an accessor.
+  // Check if this is a reference to a property or subscript accessor.
+  //
+  // Note: There is an parsing ambiguity here. An accessor label identifier
+  // (e.g. "set") may refer to the final declaration name component instead of
+  // an accessor kind.
+  //
+  // FIXME: It is wrong to backtrack parsing the entire base type if an accessor
+  // label is found. Instead, only the final component of the base type should
+  // be backtracked. It may be best to implement this in
+  // `Parser::parseTypeIdentifier`.
+  //
+  // Example: consider parsing `A.B.property.set`.
+  // Current behavior: base type is entirely backtracked.
+  // Ideal behavior: base type is parsed as `A.B`.
   if (P.Tok.is(tok::period)) {
     const Token &nextToken = P.peekToken();
     if (isAccessorLabel(nextToken).hasValue())
@@ -1123,6 +1133,22 @@ static bool parseQualifiedDeclName(Parser &P, Diag<> nameParseError,
   }
 
   // Parse an optional accessor kind.
+  //
+  // Note: there is an parsing ambiguity here.
+  //
+  // Example: `A.B.property.set` may be parsed as one of the following:
+  //
+  // 1. No accessor kind.
+  // - Base type: `A.B.property`
+  // - Declaration name: `set`
+  // - Accessor kind: <none>
+  //
+  // 2. Accessor kind exists.
+  // - Base type: `A.B`
+  // - Declaration name: `property`
+  // - Accessor kind: `set`
+  //
+  // Currently, we follow (2) because it's more useful in practice.
   if (P.Tok.is(tok::period)) {
     const Token &nextToken = P.peekToken();
     Optional<AccessorKind> kind = isAccessorLabel(nextToken);