Forbid StringConstant(null) (#23064)

Fixes #23008. Started during the Scala 3 Compiler Spree of Monday, April
28th, 2025.

`Constant` is an object in Dotty core that has several `apply`
overloads, including `Constant(String)` and `Constant(Null)`, that
create `Constant` instances with the appropriate tags:

https://github.com/scala/scala3/blob/bbe2dde1ee10adc05ceece96ab82af8a66a21880/compiler/src/dotty/tools/dotc/core/Constants.scala#L229
https://github.com/scala/scala3/blob/bbe2dde1ee10adc05ceece96ab82af8a66a21880/compiler/src/dotty/tools/dotc/core/Constants.scala#L238

It is called from the quotes reflection API with an argument `x:
String`, which always dispatched to the former overload:

https://github.com/scala/scala3/blob/9e7aab7ab8b2798175abf71737ffe822e50f9c30/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala#L2532

When this API is called from code without explicit nulls, `null` can be
passed as argument `x`. That still dispatches to the `Constant(String)`
overload, yielding a `Constant` with the wrong tag.

This PR fixes the issue by forbidding `null` as an argument of
`StringConstant`.

Author: mbovel

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, "value of 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/Quotes.scala

@@ -3640,7 +3640,10 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
 
     /** Methods of the module object `val StringConstant` */
     trait StringConstantModule { this: StringConstant.type =>
-      /** Create a constant String value */
+      /** Create a constant String value
+       *
+       *  @throw `IllegalArgumentException` if the argument is `null`
+       */
       def apply(x: String): StringConstant
       /** Match String value constant and extract its value */
       def unapply(constant: StringConstant): Some[String]

tests/neg-macros/i23008.check

@@ -0,0 +1,21 @@
+
+-- Error: tests/neg-macros/i23008/Test_2.scala:1:24 --------------------------------------------------------------------
+1 |@main def Test = Macros.buildString // error
+  |                 ^^^^^^^^^^^^^^^^^^
+  |                 Exception occurred while executing macro expansion.
+  |                 java.lang.IllegalArgumentException: requirement failed: value of StringConstant cannot be `null`
+  |                 	at scala.Predef$.require(Predef.scala:337)
+  |                 	at scala.quoted.runtime.impl.QuotesImpl$reflect$StringConstant$.apply(QuotesImpl.scala:2533)
+  |                 	at scala.quoted.runtime.impl.QuotesImpl$reflect$StringConstant$.apply(QuotesImpl.scala:2532)
+  |                 	at scala.quoted.ToExpr$StringToExpr.apply(ToExpr.scala:80)
+  |                 	at scala.quoted.ToExpr$StringToExpr.apply(ToExpr.scala:78)
+  |                 	at scala.quoted.Expr$.apply(Expr.scala:70)
+  |                 	at Macros$.buildStringCode(Macro_1.scala:9)
+  |
+  |---------------------------------------------------------------------------------------------------------------------
+  |Inline stack trace
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  |This location contains code that was inlined from Macro_1.scala:4
+4 |  inline def buildString = ${buildStringCode}
+  |                           ^^^^^^^^^^^^^^^^^^
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg-macros/i23008/Macro_1.scala

@@ -0,0 +1,11 @@
+import scala.quoted.*
+
+object Macros {
+  inline def buildString = ${buildStringCode}
+
+  def buildStringCode(using Quotes): Expr[String] = {
+    import quotes.reflect.*
+    val str: String = null
+    Expr(str)
+  }
+}

tests/neg-macros/i23008/Test_2.scala

@@ -0,0 +1 @@
+@main def Test = Macros.buildString // error