address comments

compiler/options.nim

@@ -134,6 +134,8 @@ type
       ## This requires building nim with `-d:nimHasLibFFI`
       ## which itself requires `nimble install libffi`, see #10150
       ## Note: this feature can't be localized with {.push.}
+    aliasSym,
+      ## enable `Alias` magic to alias symbols
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf

compiler/semexprs.nim

@@ -2244,7 +2244,10 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
   of mSizeOf: result =
     semSizeof(c, setMs(n, s))
   of mAlias:
-    result = semAlias(c, n, s, flags)
+    if aliasSym in c.features:
+      result = semAlias(c, n, s, flags)
+    else:
+      globalError(c.config, n.info, "requires --experimental:aliasSym")
   else:
     result = semDirectOp(c, n, flags)
 

lib/pure/sugar.nim

@@ -238,3 +238,15 @@ macro distinctBase*(T: typedesc): untyped =
   while typeSym.typeKind == ntyDistinct:
     typeSym = getTypeImpl(typeSym)[0]
   typeSym.freshIdentNodes
+
+when defined(nimHasAlias):
+  proc aliasImpl[T1, T2](name: T1, expr: T2) {.magic: "Alias".}
+
+  template `:=`*(name, expr) =
+    ## Declares `a` as alias of `expr`, which must resolve to a symbol.
+    runnableExamples:
+      echo2:=system.echo
+      echo2 "hello"
+      declared2:=system.declared
+      doAssert declared2(echo2)
+    aliasImpl(name, expr)

lib/system.nim

@@ -149,18 +149,6 @@ else:
   template runnableExamples*(body: untyped) =
     discard
 
-when defined(nimHasAlias):
-  proc aliasImpl[T1, T2](name: T1, expr: T2) {.magic: "Alias".}
-
-  template `:=`*(name, expr) =
-    ## Declares `a` as alias of `expr`, which must resolve to a symbol.
-    runnableExamples:
-      echo2:=system.echo
-      echo2 "hello"
-      declared2:=system.declared
-      doAssert declared2(echo2)
-    aliasImpl(name, expr)
-
 proc declared*(x: untyped): bool {.magic: "Defined", noSideEffect, compileTime.}
   ## Special compile-time procedure that checks whether `x` is
   ## declared. `x` has to be an identifier or a qualified identifier.

tests/magics/talias2.nim

@@ -1,4 +1,7 @@
 import std/macros
+import std/sugar
+
+{.push experimental: "aliasSym".}
 
 proc fun0(a: int): auto = $a
 template fun3(a: int): untyped = $a