Fix adaptation of constants to constant type aliases (#18360)

Fixes #18340

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -4162,7 +4162,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               || x.tag == LongTag && cls == defn.DoubleClass && x.longValue.toDouble.toLong != x.longValue
             then
               report.warning(LossyWideningConstantConversion(x.tpe, pt), tree.srcPos)
-            return adaptConstant(tree, ConstantType(converted))
+            return readapt(adaptConstant(tree, ConstantType(converted)))
         case _ =>
 
       val captured = captureWildcardsCompat(wtp, pt)

tests/neg/i18340.scala

@@ -0,0 +1,36 @@
+@main def main: Unit =
+  type T = 3f
+  val value0: T = -3.5f // error
+  val value1: T = -100500 // error
+  val value2: T = -100500L // error
+  val value3: T = -100500D // error
+  val value4: T = true // error
+  val value5: 3f = -100500 // error
+  val value6: 3f = -100500L // error
+
+  type Ti = 3
+  val value1i: Ti = -100500  // error
+  val value2i: Ti = -100500L // error
+  val value0i: Ti = -100500F // error
+  val value3i: Ti = -100500D // error
+  val value4i: Ti = true     // error
+  val value5i: 3  = -100500  // error
+  val value6i: 3  = -100500L // error
+
+  type Tl = 3L
+  val value1l: Tl = -100500  // error
+  val value2l: Tl = -100500L // error
+  val value0l: Tl = -100500F // error
+  val value3l: Tl = -100500D // error
+  val value4l: Tl = true     // error
+  val value5l: 3L = -100500  // error
+  val value6l: 3L = -100500L // error
+
+  type Td = 3D
+  val value1d: Td = -100500  // error
+  val value2d: Td = -100500L // error
+  val value0d: Td = -100500F // error
+  val value3d: Td = -100500D // error
+  val value4d: Td = true     // error
+  val value5d: 3D = -100500  // error
+  val value6d: 3D = -100500L // error