fix #25789; improve handling of distinct types (#25791)

fix #25789

This pull request addresses an issue with the `distinctBase` trait in
the Nim compiler, ensuring it correctly handles types with generic
parameters and static parameters. Additionally, it adds a new test to
cover this scenario. The most important changes are:

### Compiler logic improvements

* Updated the `evalTypeTrait` implementation for the `distinctBase`
trait in `compiler/semmagic.nim` to properly skip all relevant type
wrappers, including those with generic and static parameters, when
unwrapping distinct types. This fixes incorrect handling of types like
`distinct L[int, 100]`.

### Test coverage

* Added a new test block for bug #25789 in
`tests/metatype/ttypetraits.nim` that defines a distinct type over a
generic type with a static parameter, verifies conversions, and checks
that the `distinctBase` trait returns the correct type.

compiler/semmagic.nim

@@ -248,10 +248,13 @@ proc evalTypeTrait(c: PContext; traitCall: PNode, operand: PType, context: PSym)
     assert operand.kind == tyTuple, $operand.kind
     result = newIntNodeT(toInt128(operand.len), traitCall, c.idgen, c.graph)
   of "distinctBase":
-    var arg = operand.skipTypes({tyGenericInst})
+    var arg = operand.skipTypes(skippedTypes)
     let rec = semConstExpr(c, traitCall[2]).intVal != 0
-    while arg.kind == tyDistinct:
-      arg = arg.base.skipTypes(skippedTypes + {tyGenericInst})
+    while true:
+      let distinctArg = arg.skipTypes(skippedTypes + {tyGenericInst})
+      if distinctArg.kind != tyDistinct:
+        break
+      arg = distinctArg.base.skipTypes(skippedTypes)
       if not rec: break
     result = getTypeDescNode(c, arg, operand.owner, traitCall.info)
   of "rangeBase":

tests/metatype/ttypetraits.nim

@@ -434,3 +434,32 @@ block: # bug #24378
   type Win222[T] = typeof("foobar")
   doAssert not supportsCopyMem((int, Win222[int]))
   doAssert not supportsCopyMem(tuple[a: int, b: Win222[int]])
+
+block: # bug #25789
+  type
+    L[T; N: static int] = distinct seq[T]
+    EPF = distinct L[int, 100]
+
+  var e: EPF = EPF(L[int, 100](@[1, 2, 3]))
+
+  template classifyGeneric[T](x: T): bool =
+    when typeof(x) is L:
+      true
+    else:
+      false
+
+  template classifyConcrete[T](x: T): bool =
+    when typeof(x) is L[int, 100]:
+      true
+    else:
+      false
+
+  let viaConv = L[int, 100](e)
+  doAssert $type(viaConv) == "L[system.int, 100]"
+  doAssert classifyGeneric(viaConv)
+  doAssert classifyConcrete(viaConv)
+
+  let viaDB = distinctBase(e, recursive = false)
+  doAssert $type(viaDB) == "L[system.int, 100]"
+  doAssert classifyGeneric(viaDB)
+  doAssert classifyConcrete(viaDB)