Fix #8186: Check trait secondary constructors

Author: nicolasstucki

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

@@ -440,6 +440,8 @@ object Checking {
     }
     if (sym.isValueClass && sym.is(Trait) && !sym.isRefinementClass)
       fail(CannotExtendAnyVal(sym))
+    if (sym.isConstructor && !sym.isPrimaryConstructor && sym.owner.is(Trait))
+      fail("Traits cannot have secondary constructors")
     checkCombination(Final, Open)
     checkCombination(Sealed, Open)
     checkCombination(Final, Sealed)

tests/neg/i8186.scala

@@ -0,0 +1,9 @@
+trait Test1() {
+  def this(x: Int) = // error
+    this()
+}
+
+trait Test2(x: Int) {
+  def this() = // error
+    this(3)
+}