Fix #5004: Turn assertion into test

Author: odersky

compiler/src/dotty/tools/dotc/core/Contexts.scala

@@ -378,7 +378,6 @@ object Contexts {
      *   - as scope: The parameters of the auxiliary constructor.
      */
     def thisCallArgContext: Context = {
-      assert(owner.isClassConstructor)
       val constrCtx = outersIterator.dropWhile(_.outer.owner == owner).next()
       superOrThisCallContext(owner, constrCtx.scope)
         .setTyperState(typerState)

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

@@ -747,7 +747,8 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
    *  otherwise the current context.
    */
   def argCtx(app: untpd.Tree)(implicit ctx: Context): Context =
-    if (untpd.isSelfConstrCall(app)) ctx.thisCallArgContext else ctx
+    if (ctx.owner.isClassConstructor && untpd.isSelfConstrCall(app)) ctx.thisCallArgContext
+    else ctx
 
   /** Typecheck application. Result could be an `Apply` node,
    *  or, if application is an operator assignment, also an `Assign` or

tests/neg/i5004.scala

@@ -0,0 +1,6 @@
+object i0 {
+1 match {
+def this(): Int  // error // error
+  def this()     // error
+}                // error
+}