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

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

compiler/semfold.nim

@@ -674,7 +674,7 @@ proc getConstExpr(m: PSym, n: PNode; idgen: IdGenerator; g: ModuleGraph): PNode
   of nkCast:
     var a = getConstExpr(m, n[1], idgen, g)
     if a == nil: return
-    if n.typ != nil and n.typ.kind in NilableTypes:
+    if n.typ != nil and n.typ.kind in NilableTypes and a.kind != nkNilLit:
       # we allow compile-time 'cast' for pointer types:
       result = a
       result.typ = n.typ

compiler/vmgen.nim

@@ -901,6 +901,11 @@ proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
     c.gABx(n, opcSetType, dest, c.genType(dst))
     c.gABC(n, opcCastIntToPtr, dest, tmp)
     c.freeTemp(tmp)
+  elif src.kind == tyNil and dst.kind in NilableTypes:
+    # supports casting nil literals to NilableTypes in VM
+    # see #16024
+    if dest < 0: dest = c.getTemp(n[0].typ)
+    genLit(c, n[1], dest)
   else:
     # todo: support cast from tyInt to tyRef
     globalError(c.config, n.info, "VM does not support 'cast' from " & $src.kind & " to " & $dst.kind)

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"