combine PR#16009 and PR#16012 (#16024)

* fix #15623
* add testcase for #15623
* add testcase
* combine #16009
* support casting nil to NilableTypes in VM

tests/misc/tcast.nim

@@ -29,3 +29,47 @@ reject: discard cast[enum](0)
 proc a = echo "hi"
 
 reject: discard cast[ptr](a)
+
+# bug #15623
+block:
+  if false:
+    echo cast[ptr int](nil)[]
+
+block:
+  if false:
+    var x: ref int = nil
+    echo cast[ptr int](x)[]
+
+block:
+  doAssert cast[int](cast[ptr int](nil)) == 0
+
+block:
+  var x: ref int = nil
+  doAssert cast[int](cast[ptr int](x)) == 0
+
+block:
+  block:
+    static:
+      let a = cast[pointer](nil)
+      doAssert a.repr == "nil"
+
+  block:
+    static:
+      doAssert cast[ptr int](nil).repr == "nil"
+
+  block:
+    const str = cast[ptr int](nil)
+    static:
+      doAssert str.repr == "nil"
+
+  block:
+    static:
+      doAssert cast[ptr int](nil).repr == "nil"
+
+  block:
+    static:
+      doAssert cast[RootRef](nil).repr == "nil"
+
+  block:
+    static:
+      doAssert cast[cstring](nil).repr == "nil"