fix #7535(Poor error message for spawn when a procedure (without calling it)) (#17774)

compiler/semexprs.nim

@@ -2282,12 +2282,15 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
   of mSpawn:
     markUsed(c, n.info, s)
     when defined(leanCompiler):
-      localError(c.config, n.info, "compiler was built without 'spawn' support")
-      result = n
+      result = localErrorNode(c, n, "compiler was built without 'spawn' support")
     else:
       result = setMs(n, s)
       for i in 1..<n.len:
         result[i] = semExpr(c, n[i])
+
+      if n.len > 1 and n[1].kind notin nkCallKinds:
+        return localErrorNode(c, n, n[1].info, "'spawn' takes a call expression; got " & $n[1])
+
       let typ = result[^1].typ
       if not typ.isEmptyType:
         if spawnResult(typ, c.inParallelStmt > 0) == srFlowVar:

tests/parallel/t7535.nim

@@ -0,0 +1,11 @@
+discard """
+  matrix: "--threads:on"
+  errormsg: "'spawn' takes a call expression; got proc (x: uint32) = echo [x]"
+"""
+
+import threadpool
+
+# bug #7535
+proc print_parallel_nok(r: uint32) =
+  for x in 0..r:
+    spawn (proc (x: uint32) = echo x)