trigger range check with new type inference on nkIntLit [backport:1.6] (#23179)

fixes #23177

`changeType` doesn't perform range checks to see if the expression fits
the new type [if the old type is the same as the new
type](62d8ca4306/compiler/semexprs.nim (L633)).
For `nkIntLit`, we previously set the type to the concrete base of the
expected type first, then call `changeType`, which works for things like
range types but not bare types of smaller bit size like `int8`. Now we
don't set the type (so the type is nil), and `changeType` performs the
range check when the type is unset (nil).

compiler/semexprs.nim

@@ -630,7 +630,7 @@ proc changeType(c: PContext; n: PNode, newType: PType, check: bool) =
           a.add m
           changeType(m, tup[i], check)
   of nkCharLit..nkUInt64Lit:
-    if check and n.kind != nkUInt64Lit and not sameType(n.typ, newType):
+    if check and n.kind != nkUInt64Lit and not sameTypeOrNil(n.typ, newType):
       let value = n.intVal
       if value < firstOrd(c.config, newType) or value > lastOrd(c.config, newType):
         localError(c.config, n.info, "cannot convert " & $value &
@@ -3152,7 +3152,6 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}, expectedType: PType
           expected.kind in {tyInt..tyInt64,
             tyUInt..tyUInt64,
             tyFloat..tyFloat128}):
-        result.typ = expected
         if expected.kind in {tyFloat..tyFloat128}:
           n.transitionIntToFloatKind(nkFloatLit)
         changeType(c, result, expectedType, check=true)

tests/overflow/twronginference.nim

@@ -0,0 +1,10 @@
+discard """
+  errormsg: "cannot convert 256 to int8"
+  line: 9
+"""
+
+# issue #23177
+
+var x: int8
+x = 256
+echo x # 0