fixes #21360; discarding empty seqs/arrays now raises errors (#21374)

* discarding empty seqs now raises errors

* the same goes for sets

compiler/semstmts.nim

@@ -40,6 +40,13 @@ const
 
 proc implicitlyDiscardable(n: PNode): bool
 
+proc hasEmpty(typ: PType): bool =
+  if typ.kind in {tySequence, tyArray, tySet}:
+    result = typ.lastSon.kind == tyEmpty
+  elif typ.kind == tyTuple:
+    for s in typ.sons:
+      result = result or hasEmpty(s)
+
 proc semDiscard(c: PContext, n: PNode): PNode =
   result = n
   checkSonsLen(n, 1, c.config)
@@ -47,7 +54,9 @@ proc semDiscard(c: PContext, n: PNode): PNode =
     n[0] = semExprWithType(c, n[0])
     let sonType = n[0].typ
     let sonKind = n[0].kind
-    if isEmptyType(sonType) or sonType.kind in {tyNone, tyTypeDesc} or sonKind == nkTypeOfExpr:
+    if isEmptyType(sonType) or hasEmpty(sonType) or
+          sonType.kind in {tyNone, tyTypeDesc} or
+          sonKind == nkTypeOfExpr:
       localError(c.config, n.info, errInvalidDiscard)
     if sonType.kind == tyProc and sonKind notin nkCallKinds:
       # tyProc is disallowed to prevent ``discard foo`` to be valid, when ``discard foo()`` is meant.
@@ -410,13 +419,6 @@ proc semUsing(c: PContext; n: PNode): PNode =
     if a[^1].kind != nkEmpty:
       localError(c.config, a.info, "'using' sections cannot contain assignments")
 
-proc hasEmpty(typ: PType): bool =
-  if typ.kind in {tySequence, tyArray, tySet}:
-    result = typ.lastSon.kind == tyEmpty
-  elif typ.kind == tyTuple:
-    for s in typ.sons:
-      result = result or hasEmpty(s)
-
 proc hasUnresolvedParams(n: PNode; flags: TExprFlags): bool =
   result = tfUnresolved in n.typ.flags
   when false:

tests/discard/tillegaldiscardtypes.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim check $file"
+  errormsg: "statement returns no value that can be discarded"
+  nimout: '''
+tillegaldiscardtypes.nim(11, 3) Error: statement returns no value that can be discarded
+tillegaldiscardtypes.nim(12, 3) Error: statement returns no value that can be discarded
+'''
+"""
+
+proc b(v: int) = # bug #21360
+  discard @[]
+  discard []
+
+b(0)

tests/typerel/ttypelessemptyset.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "internal error: invalid kind for lastOrd(tyEmpty)"
+  errormsg: "statement returns no value that can be discarded"
 """
 var q = false
 discard (if q: {} else: {})