SI-9314 Ignore "${}"

As an Easter egg, let "${} $x" forego the check on `x`.

In other words, empty expression interpolation looks too
degenerate to check.

2 files changed, 7 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 3f0d0e655d..1b55618691 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -105,7 +105,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
   // that are turned private by typedBlock
   private final val SYNTHETIC_PRIVATE = TRANS_FLAG
 
-  private final val InterpolatorCodeRegex  = """\$\{.*?\}""".r
+  private final val InterpolatorCodeRegex  = """\$\{(.*?)\}""".r
   private final val InterpolatorIdentRegex = """\$[$\w]+""".r // note that \w doesn't include $
 
   abstract class Typer(context0: Context) extends TyperDiagnostics with Adaptation with Tag with PatternTyper with TyperContextErrors {
@@ -5211,11 +5211,13 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
         def maybeWarn(s: String): Unit = {
           def warn(message: String)         = context.warning(lit.pos, s"possible missing interpolator: $message")
           def suspiciousSym(name: TermName) = context.lookupSymbol(name, _ => true).symbol
-          def suspiciousExpr                = InterpolatorCodeRegex findFirstIn s
+          val suspiciousExpr                = InterpolatorCodeRegex findFirstMatchIn s
           def suspiciousIdents              = InterpolatorIdentRegex findAllIn s map (s => suspiciousSym(TermName(s drop 1)))
 
           if (suspiciousExpr.nonEmpty)
-            warn("detected an interpolated expression") // "${...}"
+            suspiciousExpr filter (!_.group(1).trim.isEmpty) foreach (_ =>
+              warn("detected an interpolated expression") // "${...}"
+            )
           else
             suspiciousIdents find isPlausible foreach (sym => warn(s"detected interpolated identifier `$$${sym.name}`")) // "$id"
         }
diff --git a/test/files/neg/t7848-interp-warn.scala b/test/files/neg/t7848-interp-warn.scala
index 6a289b2b71..635dd48c27 100644
--- a/test/files/neg/t7848-interp-warn.scala
+++ b/test/files/neg/t7848-interp-warn.scala
@@ -25,4 +25,6 @@ object Test {
   def i = s"Try using '${ "$bar" }' instead."                 // was: no warn on space test
   def j = s"Try using '${ "something like $bar" }' instead."  // warn
   def k = f"Try using '$bar' instead."                        // no warn on other std interps
+  def p = "Template ${} {}"                                   // no warn on unlikely or empty expressions
+  def q = "${}$bar"                                           // disables subsequent checks!
 }