fixes #25608; ImplicitRangeConversion now skips compile-time constants

The warning gate previously only exempted literal AST nodes
(nkCharLit..nkUInt64Lit, nkFloatLit..nkFloat128Lit). Enum constants,
named consts, and constant expressions passed through and triggered a
spurious ImplicitRangeConversion warning even though the compiler already
knows their value and can validate range membership exactly.

Replace the literal-kind check with a call to getConstExpr: if the
source node folds to a compile-time constant the warning is suppressed.
Non-constant values (variables, parameters, runtime expressions) are
unaffected and still warn as before.

Add tests/range/timplicitrangeconsts.nim to guard the fix.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

compiler/sempass2.nim

@@ -1559,9 +1559,10 @@ proc track(tracked: PEffects, n: PNode) =
       message(tracked.config, n.info, warnPtrToCstringConv,
           $n[1].typ)
 
-    # Check for implicit range conversions
+    # Check for implicit range conversions. Compile-time constants are already
+    # fully known here, so only non-constant values need the downsizing warning.
     if n.kind == nkHiddenStdConv and (not tracked.isArrayIndexing) and
-          n[1].kind notin {nkCharLit..nkUInt64Lit, nkFloatLit..nkFloat128Lit} and
+          getConstExpr(tracked.ownerModule, n[1], tracked.c.idgen, tracked.graph) == nil and
           shouldWarnRangeConversion(tracked.config, n.info, n.typ, n[1].typ):
       message(tracked.config, n.info, warnImplicitRangeConversion,
               typeToString(n[1].typ) & " -> " & typeToString(n.typ))

tests/range/timplicitrangeconsts.nim

@@ -0,0 +1,30 @@
+discard """
+cmd: "nim check $options --hints:off --warning:ImplicitRangeConversion --warningaserror:ImplicitRangeConversion $file"
+action: "compile"
+"""
+
+type
+  E = enum
+    ea, eb
+
+  R = range[eb..eb]
+  I = range[0..3]
+
+proc accept(r: R) = discard
+proc accept(i: I) = discard
+
+var r: R
+var i: I
+const enumOk = eb
+const enumAlias = enumOk
+const intOk = 1 + 2
+
+r = eb
+r = enumOk
+r = enumAlias
+accept(eb)
+accept(enumOk)
+accept(enumAlias)
+
+i = intOk
+accept(intOk)