distinctBase type trait for distinct types (#13031)

tests/metatype/ttypetraits.nim

@@ -1,4 +1,5 @@
 import typetraits
+import macros
 
 block: # isNamedTuple
   type Foo1 = (a:1,).type
@@ -9,6 +10,7 @@ block: # isNamedTuple
   doAssert (a:1,).type.isNamedTuple
   doAssert Foo1.isNamedTuple
   doAssert Foo2.isNamedTuple
+  doAssert isNamedTuple(tuple[key: int])
   doAssert not Foo3.isNamedTuple
   doAssert not Foo4.isNamedTuple
   doAssert not (1,).type.isNamedTuple
@@ -42,3 +44,50 @@ block: # typeToString
   doAssert (tuple[a: C2b[MyInt, C4[cstring]], b: cint, c: float]).name3 ==
     "tuple[a: C2b{C}[MyInt{int}, C4[cstring]], b: cint{int32}, c: float]"
 
+
+#----------------------------------------------------
+
+block distinctBase:
+  block:
+    type
+      Foo[T] = distinct seq[T]
+    var a: Foo[int]
+    doAssert a.type.distinctBase is seq[int]
+
+  block:
+    # simplified from https://github.com/nim-lang/Nim/pull/8531#issuecomment-410436458
+    macro uintImpl(bits: static[int]): untyped =
+      if bits >= 128:
+        let inner = getAST(uintImpl(bits div 2))
+        result = newTree(nnkBracketExpr, ident("UintImpl"), inner)
+      else:
+        result = ident("uint64")
+
+    type
+      BaseUint = UintImpl or SomeUnsignedInt
+      UintImpl[Baseuint] = object
+      Uint[bits: static[int]] = distinct uintImpl(bits)
+
+    doAssert Uint[128].distinctBase is UintImpl[uint64]
+
+    block:
+      type
+        AA = distinct seq[int]
+        BB = distinct string
+        CC = distinct int
+        AAA = AA
+
+      static:
+        var a2: AAA
+        var b2: BB
+        var c2: CC
+
+        doAssert(a2 is distinct)
+        doAssert(b2 is distinct)
+        doAssert(c2 is distinct)
+
+        doAssert($distinctBase(typeof(a2)) == "seq[int]")
+        doAssert($distinctBase(typeof(b2)) == "string")
+        doAssert($distinctBase(typeof(c2)) == "int")
+
+