Fix Constant tag of Expr(null: String)

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -2529,7 +2529,12 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
     end StringConstantTypeTest
 
     object StringConstant extends StringConstantModule:
-      def apply(x: String): StringConstant = dotc.core.Constants.Constant(x)
+      def apply(x: String): StringConstant =
+        require(x != null, "StringConstant cannot be null")
+        // A `null` constant must be represented as a `NullConstant`, c.f. a
+        // constant with `tag == NullTag`, which is not a `StringConstant`.
+        // See issue 23008.
+        dotc.core.Constants.Constant(x)
       def unapply(constant: StringConstant): Some[String] = Some(constant.stringValue)
     end StringConstant
 

library/src/scala/quoted/ToExpr.scala

@@ -77,7 +77,13 @@ object ToExpr {
   given StringToExpr[T <: String]: ToExpr[T] with {
     def apply(x: T)(using Quotes) =
       import quotes.reflect.*
-      Literal(StringConstant(x)).asExpr.asInstanceOf[Expr[T]]
+      val literal =
+        if (x: Any) == null then
+          // Can happen if called from code without `-Yexplicit-nulls`.
+          Literal(NullConstant())
+        else
+          Literal(StringConstant(x))
+      literal.asExpr.asInstanceOf[Expr[T]]
   }
 
   /** Default implementation of `ToExpr[Class[T]]` */

tests/run-macros/i23008.check

@@ -0,0 +1 @@
+null

tests/run-macros/i23008/Macros_1.scala

@@ -0,0 +1,12 @@
+import scala.quoted.*
+
+object Macros {
+  inline def buildString = ${buildStringCode}
+
+  def buildStringCode(using Quotes): Expr[String] = {
+    import quotes.reflect.*
+    val str: String = null
+    val exprString = Expr(str)
+    Expr(exprString.show)
+  }
+}

tests/run-macros/i23008/Test_2.scala

@@ -0,0 +1,2 @@
+@main def Test =
+  println(Macros.buildString)