Fix concepts with doc comments (#22228)

* Add testcase

This tries to use a concept with a doc comment which currently leads to a segfault

* Ignore nil nodes which happen when there are doc comments in new style concept

This was done instead of semming the comments since `semConceptDecl` says it only supports lists of actual statements

* Go with alternative fix: Sem comments but ignore them

Since `nil` could mean anything it is best to not silently ignore it (In case another nil problem happens in future

Also fix test case so it isn't an infinite loop

compiler/concepts.nim

@@ -62,7 +62,7 @@ proc semConceptDecl(c: PContext; n: PNode): PNode =
       result[i] = n[i]
     result[^1] = semConceptDecl(c, n[^1])
   of nkCommentStmt:
-    discard
+    result = n
   else:
     localError(c.config, n.info, "unexpected construct in the new-styled concept: " & renderTree(n))
     result = n
@@ -306,6 +306,8 @@ proc conceptMatchNode(c: PContext; n: PNode; m: var MatchCon): bool =
     result = matchSyms(c, n, {skMethod}, m)
   of nkIteratorDef:
     result = matchSyms(c, n, {skIterator}, m)
+  of nkCommentStmt:
+    result = true
   else:
     # error was reported earlier.
     result = false

tests/concepts/tconcepts.nim

@@ -31,6 +31,7 @@ e
 20
 10
 5
+9
 '''
 """
 
@@ -438,3 +439,13 @@ import mvarconcept
 block tvar:
   # bug #2346, bug #2404
   echo randomInt(5)
+
+block tcomment:
+  type
+    Foo = concept
+      ## Some comment
+      proc bar(x: Self)
+
+  proc bar(x: int) = echo x
+  proc foo(x: Foo) = x.bar
+  foo(9)