From d9e28aac8ec35f7b3cd00d42a8515c04e72aafed Mon Sep 17 00:00:00 2001
From: Andreas Rumpf <rumpf_a@web.de>
Date: Mon, 8 Jun 2026 11:32:04 +0200
Subject: [PATCH] parser: `concept of` (#25878)

Co-authored-by: Gerke Max Preussner <gmpreussner@headcrash.industries>
---
 compiler/parser.nim | 12 ++++++++----
 doc/grammar.txt     |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/compiler/parser.nim b/compiler/parser.nim
index 934db857b2..32bf4b3d52 100644
--- a/compiler/parser.nim
+++ b/compiler/parser.nim
@@ -2241,14 +2241,17 @@ proc parseTypeClassParam(p: var Parser): PNode =
 
 proc parseTypeClass(p: var Parser): PNode =
   #| conceptParam = ('var' | 'out' | 'ptr' | 'ref' | 'static' | 'type')? symbol
-  #| conceptDecl = 'concept' conceptParam ^* ',' (pragma)? ('of' typeDesc ^* ',')?
+  #| conceptDecl = 'concept' (conceptParam ^* ',' (pragma)?)? ('of' typeDesc ^* ',')?
   #|               &IND{>} stmt
   result = newNodeP(nkTypeClassTy, p)
   getTok(p)
   if p.tok.tokType == tkComment:
     skipComment(p, result)
 
-  if p.tok.indent < 0:
+  if p.tok.tokType == tkOf and p.tok.indent < 0:
+    # new-styled `concept of A, B` on the same line as `concept`
+    result.add(p.emptyNode)
+  elif p.tok.indent < 0:
     var args = newNodeP(nkArgList, p)
     result.add(args)
     args.add(p.parseTypeClassParam)
@@ -2274,9 +2277,10 @@ proc parseTypeClass(p: var Parser): PNode =
     result.add(p.emptyNode)
   if p.tok.tokType == tkComment:
     skipComment(p, result)
-  # an initial IND{>} HAS to follow:
+  # an initial IND{>} HAS to follow, unless this concept inherits requirements:
   if not realInd(p):
-    if result.isNewStyleConcept:
+    let hasParents = result[2].kind != nkEmpty
+    if result.isNewStyleConcept and not hasParents:
       parMessage(p, "routine expected, but found '$1' (empty new-styled concepts are not allowed)", p.tok)
     result.add(p.emptyNode)
   else:
diff --git a/doc/grammar.txt b/doc/grammar.txt
index 7d430019b1..a25ace34d6 100644
--- a/doc/grammar.txt
+++ b/doc/grammar.txt
@@ -188,7 +188,7 @@ objectPart = IND{>} objectPart^+IND{=} DED
            / objectWhen / objectCase / 'nil' / 'discard' / declColonEquals
 objectDecl = 'object' ('of' typeDesc)? COMMENT? objectPart
 conceptParam = ('var' | 'out' | 'ptr' | 'ref' | 'static' | 'type')? symbol
-conceptDecl = 'concept' conceptParam ^* ',' (pragma)? ('of' typeDesc ^* ',')?
+conceptDecl = 'concept' (conceptParam ^* ',' (pragma)?)? ('of' typeDesc ^* ',')?
               &IND{>} stmt
 typeDef = identVisDot genericParamList? pragma '=' optInd typeDefValue
             indAndComment?