don't allow casting to non-concrete types; fixes #5428 (#5502)

tests/errmsgs/tnon_concrete_cast.nim

@@ -0,0 +1,47 @@
+discard """
+  errormsg: "cannot cast to a non concrete type: 'ptr SomeNumber'"
+  line: 36
+"""
+
+# https://github.com/nim-lang/Nim/issues/5428
+
+type
+  MemFile = object
+    mem: pointer
+
+proc memfileopen(filename: string, newFileSize: int): MemFile =
+  # just a memfile mock
+  return
+
+type
+  MyData = object
+    member1: seq[int]
+    member2: int
+
+type
+  MyReadWrite = object
+    memfile: MemFile
+    offset: int
+
+# Here, SomeNumber is bound to a concrete type, and that's OK
+proc write(rw: var MyReadWrite; value: SomeNumber): void =
+  (cast[ptr SomeNumber](cast[uint](rw.memfile.mem) + rw.offset.uint))[] = value
+  rw.offset += sizeof(SomeNumber)
+
+# Here, we try to use SomeNumber without binding it to a type. This should
+# produce an error message for now. It's also possible to relax the rules
+# and allow for type-class based type inference in such situations.
+proc write[T](rw: var MyReadWrite; value: seq[T]): void =
+  rw.write value.len
+  let dst  = cast[ptr SomeNumber](cast[uint](rw.memfile.mem) + uint(rw.offset))
+  let src  = cast[pointer](value[0].unsafeAddr)
+  let size = sizeof(T) * value.len
+  copyMem(dst, src, size)
+  rw.offset += size
+
+proc saveBinFile(arg: var MyData, filename: string): void =
+  var rw: MyReadWrite
+  rw.memfile = memfileOpen(filename, newFileSize = rw.offset)
+  rw.offset = 0
+  rw.write arg.member1
+