Reject casts to builtin typeclasses (#14788)

* Closes #14231, closes #14452

* Merge test to tcast.nim

compiler/semexprs.nim

@@ -191,6 +191,8 @@ proc isCastable(conf: ConfigRef; dst, src: PType): bool =
     return false
   if skipTypes(src, abstractInst-{tyTypeDesc}).kind == tyTypeDesc:
     return false
+  if skipTypes(dst, abstractInst).kind == tyBuiltInTypeClass:
+    return false
   if conf.selectedGC in {gcArc, gcOrc}:
     let d = skipTypes(dst, abstractInst)
     let s = skipTypes(src, abstractInst)

tests/misc/tcast.nim

@@ -10,15 +10,22 @@ type MyProc3 = proc() #{.closure.} is implicit
 
 proc testProc()  = echo "Hello World"
 
+template reject(x) = doAssert(not compiles(x))
+
 proc callPointer(p: pointer) =
   # can cast to proc(){.cdecl.}
   let ffunc0 = cast[MyProc](p)
   # can cast to proc(){.nimcall.}
   let ffunc1 = cast[MyProc2](p)
   # cannot cast to proc(){.closure.}
-  doAssert(not compiles(cast[MyProc3](p)))
+  reject: cast[MyProc3](p)
 
   ffunc0()
   ffunc1()
 
 callPointer(cast[pointer](testProc))
+
+reject: discard cast[enum](0)
+proc a = echo "hi"
+
+reject: discard cast[ptr](a)