Fixes #6689 (#8135)

compiler/vm.nim

@@ -1211,8 +1211,14 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
     of opcIsNil:
       decodeB(rkInt)
       let node = regs[rb].node
-      regs[ra].intVal = ord(node.kind == nkNilLit or
+      regs[ra].intVal = ord(
-        (node.kind in {nkStrLit..nkTripleStrLit} and node.strVal.isNil))
+        # Note that `nfIsRef` + `nkNilLit` represents an allocated
+        # reference with the value `nil`, so `isNil` should be false!
+        (node.kind == nkNilLit and nfIsRef notin node.flags) or
+        (node.kind in {nkStrLit..nkTripleStrLit} and node.strVal.isNil) or
+        (not node.typ.isNil and node.typ.kind == tyProc and
+          node.typ.callConv == ccClosure and node.sons[0].kind == nkNilLit and
+          node.sons[1].kind == nkNilLit))
     of opcNBindSym:
       decodeBx(rkNode)
       regs[ra].node = copyTree(c.constants.sons[rbx])

tests/vm/tvmmisc.nim

@@ -91,6 +91,21 @@ block:
       result = size
     doAssert f(4) == 4
 
+# #6689
+block:
+  static:
+    proc foo(): int = 0
+    var f: proc(): int
+    doAssert f.isNil
+    f = foo
+    doAssert(not f.isNil)
+
+block:
+  static:
+    var x: ref ref int
+    new(x)
+    doAssert(not x.isNil)
+
 # #7871
 static:
   type Obj = object