add typetraits.pointerBase to return T in ref T|ptr T (#18293)

* add typetraits.deref to return T in ref T|ptr T

* deref => refBase

* refBase=>pointerBase

* [skip ci] address comment

lib/pure/typetraits.nim

@@ -100,6 +100,16 @@ proc isNamedTuple*(T: typedesc): bool {.magic: "TypeTrait".} =
     doAssert not isNamedTuple((string, int))
     doAssert isNamedTuple(tuple[name: string, age: int])
 
+template pointerBase*[T](_: typedesc[ptr T | ref T]): typedesc =
+  ## Returns `T` for `ref T | ptr T`.
+  runnableExamples:
+    assert (ref int).pointerBase is int
+    type A = ptr seq[float]
+    assert A.pointerBase is seq[float]
+    assert (ref A).pointerBase is A # not seq[float]
+    assert (var s = "abc"; s[0].addr).typeof.pointerBase is char
+  T
+
 proc distinctBase*(T: typedesc): typedesc {.magic: "TypeTrait".} =
   ## Returns the base type for distinct types, or the type itself otherwise.
   ##