Avoid generating named tuple for special keywords

src/Parsers/isUnquotedIdentifier.cpp

@@ -1,5 +1,6 @@
 #include <Parsers/isUnquotedIdentifier.h>
 
+#include <Parsers/CommonParsers.h>
 #include <Parsers/Lexer.h>
 
 namespace DB
@@ -7,6 +8,18 @@ namespace DB
 
 bool isUnquotedIdentifier(const String & name)
 {
+    auto is_keyword = [&name](Keyword keyword)
+    {
+        auto s = toStringView(keyword);
+        if (name.size() != s.size())
+            return false;
+        return strncasecmp(s.data(), name.data(), s.size()) == 0;
+    };
+
+    /// Special keywords are parsed as literals instead of identifiers.
+    if (is_keyword(Keyword::NULL_KEYWORD) || is_keyword(Keyword::TRUE_KEYWORD) || is_keyword(Keyword::FALSE_KEYWORD))
+        return false;
+
     Lexer lexer(name.data(), name.data() + name.size());
 
     auto maybe_ident = lexer.nextToken();

src/Parsers/isUnquotedIdentifier.h

@@ -5,6 +5,14 @@
 namespace DB
 {
 
+/// Checks if the input string @name is a valid unquoted identifier.
+///
+/// Example Usage:
+///   abc     -> true   (valid unquoted identifier)
+///   123     -> false  (identifiers cannot start with digits)
+///   `123`   -> false  (quoted identifiers are not considered)
+///   `abc`   -> false  (quoted identifiers are not considered)
+///   null    -> false  (reserved literal keyword)
 bool isUnquotedIdentifier(const String & name);
 
 }

tests/queries/0_stateless/02890_named_tuple_functions.reference

@@ -7,3 +7,4 @@ Tuple(\n    k UInt8,\n    j Int32)
 Tuple(Int32, Int32, Int32, Int32)
 ['1','2','3','4']
 (1,2,3)
+Tuple(Nullable(Nothing))	Tuple(Bool)	Tuple(Bool)

tests/queries/0_stateless/02890_named_tuple_functions.sql

@@ -28,4 +28,7 @@ create table tbl (x Tuple(a Int32, b Int32, c Int32)) engine MergeTree order by
 insert into tbl values (tuple(1, 2, 3)); -- without tuple it's interpreted differently inside values block.
 select * from tbl;
 
-drop table tbl
+drop table tbl;
+
+-- Avoid generating named tuple for special keywords
+select toTypeName(tuple(null)), toTypeName(tuple(true)), toTypeName(tuple(false));