fixes #3158

compiler/sem.nim

@@ -171,11 +171,15 @@ proc newSymS(kind: TSymKind, n: PNode, c: PContext): PSym =
   result = newSym(kind, considerQuotedIdent(n), getCurrOwner(), n.info)
 
 proc newSymG*(kind: TSymKind, n: PNode, c: PContext): PSym =
+  proc `$`(kind: TSymKind): string = substr(system.`$`(kind), 2).toLower
+
   # like newSymS, but considers gensym'ed symbols
   if n.kind == nkSym:
     # and sfGenSym in n.sym.flags:
     result = n.sym
-    internalAssert result.kind == kind
+    if result.kind != kind:
+      localError(n.info, "cannot use symbol of kind '" &
+                 $result.kind & "' as a '" & $kind & "'")
     # when there is a nested proc inside a template, semtmpl
     # will assign a wrong owner during the first pass over the
     # template; we must fix it here: see #909

tests/template/twrongsymkind.nim

@@ -0,0 +1,20 @@
+discard """
+  errormsg: "cannot use symbol of kind 'var' as a 'param'"
+  line: 20
+"""
+
+# bug #3158
+
+type
+  MyData = object
+      x: int
+
+template newDataWindow(data: ref MyData): stmt =
+    proc testProc(data: ref MyData) =
+        echo "Hello, ", data.x
+    testProc(data)
+
+var d: ref MyData
+new(d)
+d.x = 10
+newDataWindow(d)