distinctBase now is identity instead of error for non distinct types (#16891)

changelog.md

@@ -119,6 +119,7 @@ with other backends. see #9125. Use `-d:nimLegacyJsRound` for previous behavior.
 
 - nil dereference is not allowed at compile time. `cast[ptr int](nil)[]` is rejected at compile time.
 
+- `typetraits.distinctBase` now is identity instead of error for non distinct types.
 
 ## Compiler changes
 

compiler/semmagic.nim

@@ -190,15 +190,10 @@ proc evalTypeTrait(c: PContext; traitCall: PNode, operand: PType, context: PSym)
     result = newIntNodeT(toInt128(operand.len), traitCall, c.idgen, c.graph)
   of "distinctBase":
     var arg = operand.skipTypes({tyGenericInst})
-    if arg.kind == tyDistinct:
     while arg.kind == tyDistinct:
       arg = arg.base
       arg = arg.skipTypes(skippedTypes + {tyGenericInst})
     result = getTypeDescNode(c, arg, operand.owner, traitCall.info)
-    else:
-      localError(c.config, traitCall.info,
-        "distinctBase expects a distinct type as argument. The given type was " & typeToString(operand))
-      result = newType(tyError, nextTypeId c.idgen, context).toNode(traitCall.info)
   else:
     localError(c.config, traitCall.info, "unknown trait: " & s)
     result = newNodeI(nkEmpty, traitCall.info)

lib/pure/typetraits.nim

@@ -86,26 +86,26 @@ proc isNamedTuple*(T: typedesc): bool {.magic: "TypeTrait".} =
     doAssert isNamedTuple(tuple[name: string, age: int])
 
 proc distinctBase*(T: typedesc): typedesc {.magic: "TypeTrait".} =
-  ## Returns the base type for distinct types. This works only
+  ## Returns the base type for distinct types, or the type itself otherwise.
-  ## for distinct types and produces a compile time error otherwise.
   ##
   ## **See also:**
   ## * `distinctBase template <#distinctBase.t,T>`_
   runnableExamples:
     type MyInt = distinct int
-
     doAssert distinctBase(MyInt) is int
-    doAssert not compiles(distinctBase(int))
+    doAssert distinctBase(int) is int
 
 since (1, 1):
   template distinctBase*[T](a: T): untyped =
     ## Overload of `distinctBase <#distinctBase,typedesc>`_ for values.
     runnableExamples:
       type MyInt = distinct int
-
       doAssert 12.MyInt.distinctBase == 12
-
+      doAssert 12.distinctBase == 12
+    when T is distinct:
       distinctBase(type(a))(a)
+    else: # avoids hint ConvFromXtoItselfNotNeeded
+      a
 
   proc tupleLen*(T: typedesc[tuple]): int {.magic: "TypeTrait".} =
     ## Returns the number of elements of the tuple type `T`.

tests/metatype/ttypetraits.nim

@@ -90,6 +90,8 @@ block distinctBase:
       Foo[T] = distinct seq[T]
     var a: Foo[int]
     doAssert a.type.distinctBase is seq[int]
+    doAssert seq[int].distinctBase is seq[int]
+    doAssert "abc".distinctBase == "abc"
 
   block:
     # simplified from https://github.com/nim-lang/Nim/pull/8531#issuecomment-410436458