Allowing nil for distinct types where the base type is nilable

tests/distinct/tnil.nim

@@ -0,0 +1,47 @@
+discard """
+  file: "tnil.nim"
+  output: '''0x1
+
+nil
+
+nil
+
+'''
+"""
+
+type
+  MyPointer = distinct pointer
+  MyString = distinct string
+  MyStringNotNil = distinct (string not nil)
+  MyInt = distinct int
+
+proc foo(a: MyPointer) =
+  echo a.repr
+
+foo(cast[MyPointer](1))
+foo(cast[MyPointer](nil))
+foo(nil)
+
+var p: MyPointer
+p = cast[MyPointer](1)
+p = cast[MyPointer](nil)
+p = nil.MyPointer
+p = nil
+
+var c: MyString
+c = "Test".MyString
+c = nil.MyString
+c = nil
+
+p = nil
+doAssert(compiles(c = p) == false)
+
+var n: MyStringNotNil = "Test".MyStringNotNil # Cannot prove warning ...
+n = "Test".MyStringNotNil
+doAssert(compiles(n = nil.MyStringNotNil) == false)
+doAssert(compiles(n = nil.MyStringNotNil) == false)
+doAssert(compiles(n = nil) == false)
+
+var i: MyInt
+i = 1.MyInt
+doAssert(compiles(i = nil) == false)