Fix #4732: Root the desugared call to StringContext

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1109,8 +1109,8 @@ object desugar {
           case Block(Nil, expr) => expr // important for interpolated string as patterns, see i1773.scala
           case t => t
         }
-
-        Apply(Select(Apply(Ident(nme.StringContext), strs), id), elems)
+        // This is a deliberate departure from scalac, where StringContext is not rooted (See #4732)
+        Apply(Select(Apply(scalaDot(nme.StringContext), strs), id), elems)
       case InfixOp(l, op, r) =>
         if (ctx.mode is Mode.Type)
           AppliedTypeTree(op, l :: r :: Nil) // op[l, r]

compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala

@@ -34,10 +34,8 @@ class StringInterpolatorOpt extends MiniPhase {
 
   private object StringContextApply {
     def unapply(tree: Select)(implicit ctx: Context): Boolean = {
-      tree.symbol.eq(defn.StringContextModule_apply) && {
-        val qualifier = tree.qualifier
-        qualifier.isInstanceOf[Ident] && qualifier.symbol.eq(defn.StringContextModule)
-      }
+      tree.symbol.eq(defn.StringContextModule_apply) &&
+      tree.qualifier.symbol.eq(defn.StringContextModule)
     }
   }
 

tests/run/rooted_stringcontext.scala

@@ -0,0 +1,10 @@
+object StringContext {
+  val i = 42
+  val s = s"Answer: $i"
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(StringContext.s == "Answer: 42")
+  }
+}