diff --git a/src/data/error.rs b/src/data/error.rs
index 89894721..b9480ed0 100644
--- a/src/data/error.rs
+++ b/src/data/error.rs
@@ -510,6 +510,10 @@ pub enum CppError {
     /// '##' missing arguments
     #[error("'##' cannot appear at {} of macro expansion", if *(.0) { "start" } else { "end"})]
     HashHashMissingParameter(bool),
+
+    /// The result of '##' is not a valid token
+    #[error("pasting formed '{0}{1}', an invalid preprocessing token")]
+    HashHashInvalid(Token, Token),
 }
 
 /// Lex errors are non-exhaustive and may have new variants added at any time
diff --git a/src/data/lex.rs b/src/data/lex.rs
index 3680846d..e609f626 100644
--- a/src/data/lex.rs
+++ b/src/data/lex.rs
@@ -299,6 +299,54 @@ impl<T> Locatable<T> {
 
 impl Token {
     pub const EQUAL: Token = Token::Assignment(AssignmentToken::Equal);
+
+    pub fn concat(x: &Token, y: &Token) -> Option<Token> {
+        use Token::*;
+        use AssignmentToken::*;
+        use ComparisonToken::*;
+        let tok = match (x, y) {
+            (Whitespace(wx), Whitespace(wy)) => Whitespace(format!("{}{}", wx, wy)),
+            (Whitespace(_), tok) => tok.clone(),
+            (tok, Whitespace(_)) => tok.clone(),
+
+            (Plus, Assignment(Equal)) => Assignment(AddEqual),
+            (Minus, Assignment(Equal)) => Assignment(SubEqual),
+            (Star, Assignment(Equal)) => Assignment(MulEqual),
+            (Divide, Assignment(Equal)) => Assignment(DivEqual),
+            (Mod, Assignment(Equal)) => Assignment(ModEqual),
+            (ShiftLeft, Assignment(Equal)) => Assignment(ShlEqual),
+            (ShiftRight, Assignment(Equal)) => Assignment(ShrEqual),
+            (Ampersand, Assignment(Equal)) => Assignment(AndEqual),
+            (BitwiseOr, Assignment(Equal)) => Assignment(OrEqual),
+            (Xor, Assignment(Equal)) => Assignment(XorEqual),
+
+            (Assignment(Equal), Assignment(Equal)) => Comparison(EqualEqual),
+            (LogicalNot, Assignment(Equal)) => Comparison(NotEqual),
+            (Comparison(Less), Assignment(Equal)) => Comparison(LessEqual),
+            (Comparison(Greater), Assignment(Equal)) => Comparison(GreaterEqual),
+
+            (Ampersand, Ampersand) => LogicalAnd,
+            (BitwiseOr, BitwiseOr) => LogicalOr,
+
+            (Comparison(Less), Comparison(Less)) => ShiftLeft,
+            (Comparison(Greater), Comparison(Greater)) => ShiftRight,
+
+            (Minus, Comparison(Greater)) => StructDeref,
+            (Hash, Hash) => HashHash,
+
+            (Keyword(kx), Keyword(ky)) => Id(InternedStr::get_or_intern(format!("{}{}", kx, ky))),
+            (Keyword(kx), Id(idy)) => Id(InternedStr::get_or_intern(format!("{}{}", kx, idy))),
+            (Id(idx), Keyword(ky)) => Id(InternedStr::get_or_intern(format!("{}{}", idx, ky))),
+            (Id(idx), Id(idy)) => Id(InternedStr::get_or_intern(format!("{}{}", idx, idy))),
+
+            // Chars and Strings cannot be concatenated with token pasting
+            // TODO implement Literals
+
+            // TODO what about Keyword::UserTypedef as differentiated from Id?
+            _ => return None,
+        };
+        Some(tok)
+    }
 }
 
 impl AssignmentToken {
diff --git a/src/lex/replace.rs b/src/lex/replace.rs
index 3ccb72ec..436ec69e 100644
--- a/src/lex/replace.rs
+++ b/src/lex/replace.rs
@@ -183,7 +183,17 @@ pub fn replace(
                     continue;
                 }
                 let pending_hashhash = pending_hashhash.take().unwrap(); // We just checked that it's some
-                let concat_token = concat(pending_hashhash, succeeding_tok.clone(), &location);
+                let concat_token = Token::concat(&pending_hashhash, succeeding_tok)
+                    .map(|tok| location.with(tok))
+                    .ok_or_else(|| {
+                        location.with(
+                            CppError::HashHashInvalid(
+                                pending_hashhash.clone(),
+                                succeeding_tok.clone(),
+                            )
+                            .into(),
+                        )
+                    });
                 replacements.push(concat_token); // TODO don't bypass pending
                 continue;
             }
@@ -480,10 +490,6 @@ fn stringify(args: Vec<Token>) -> Token {
     ))]))
 }
 
-fn concat(x: Token, b: Token, location: &Location) -> CompileResult<Locatable<Token>> {
-    todo!();
-}
-
 fn wrap_error(location: &Location, err: CppError) -> Vec<CppResult<Token>> {
     vec![Err(location.with(err.into()))]
 }