Improve requires clause parsing

Author: KangarooKoala

cxxheaderparser/parser.py

@@ -803,13 +803,6 @@ def _parse_concept(
             ),
         )
 
-    # fmt: off
-    _expr_operators = {
-        "<", ">", "|", "%", "^", "!", "*", "-", "+", "&", "=",
-        "&&", "||", "<<"
-    }
-    # fmt: on
-
     def _parse_requires(
         self,
         tok: LexToken,
@@ -818,38 +811,48 @@ def _parse_requires(
 
         rawtoks: typing.List[LexToken] = []
 
-        # The easier case -- requires requires
-        if tok.type == "requires":
-            rawtoks.append(tok)
-            for tt in ("(", "{"):
-                tok = self._next_token_must_be(tt)
+        # The expression in a requires clause must be one of the following:
+        #  1) A primary expression
+        #  2) A sequence of (1) joined with &&
+        #  3) A sequence of (2) joined with ||
+        #
+        # In terms of validity, this is equivalent to a sequence of primary expressions
+        # joined with && and/or ||.
+        #
+        # In general, a primary expression is one of the following:
+        #  1) this
+        #  2) a literal
+        #  3) an identifier expression
+        #  4) a lambda expression
+        #  5) a fold expression
+        #  6) a requires expression
+        #  7) any parenthesized expression
+        #
+        # For simplicity, we only consider the following primary expressions:
+        #  1) parenthesized expressions (which includes fold expressions)
+        #  2) requires expressions
+        #  3) identifer expressions (possibly qualified, possibly templated)
+        while True:
+            if tok.type == "(":
                 rawtoks.extend(self._consume_balanced_tokens(tok))
-            # .. and that's it?
-
-        # this is either a parenthesized expression or a primary clause
-        elif tok.type == "(":
-            rawtoks.extend(self._consume_balanced_tokens(tok))
-        else:
-            while True:
-                if tok.type == "(":
+                tok = self.lex.token()
+            elif tok.type == "requires":
+                rawtoks.append(tok)
+                for tt in ("(", "{"):
+                    tok = self._next_token_must_be(tt)
                     rawtoks.extend(self._consume_balanced_tokens(tok))
-                else:
-                    tok = self._parse_requires_segment(tok, rawtoks)
+                tok = self.lex.token()
+            else:
+                tok = self._parse_requires_segment(tok, rawtoks)
 
-                # If this is not an operator of some kind, we don't know how
-                # to proceed so let the next parser figure it out
-                if tok.value not in self._expr_operators:
-                    break
+            if tok.value not in ("&&", "||"):
+                break
 
-                rawtoks.append(tok)
+            rawtoks.append(tok)
 
-                # check once more for compound operator?
-                tok = self.lex.token()
-                if tok.value in self._expr_operators:
-                    rawtoks.append(tok)
-                    tok = self.lex.token()
+            tok = self.lex.token()
 
-            self.lex.return_token(tok)
+        self.lex.return_token(tok)
 
         return self._create_value(rawtoks)