error when unrolling trait constructors

Traits are forbidden in source code from having secondary constructors,
which is what the current transform would generate.

Trait parameters are encoded in concrete implementing classes as getter methods,
perhaps unroll could provide default implementations, but this is unexplored.

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -137,7 +137,11 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         if method.is(Deferred) then
           report.error("Unrolled method must be final and concrete", method.srcPos)
           res = false
-        if !(method.isConstructor || method.is(Final) || method.owner.is(ModuleClass)) then
+        val isCtor = method.isConstructor
+        if isCtor && method.owner.is(Trait) then
+          report.error("implementation restriction: Unrolled method cannot be a trait constructor", method.srcPos)
+          res = false
+        if !(isCtor || method.is(Final) || method.owner.is(ModuleClass)) then
           report.error("Unrolled method must be final", method.srcPos)
           res = false
         res

tests/neg/unroll-traitConstructor.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/unroll-traitConstructor.scala:5:12 -----------------------------------------------------------------
+5 |trait Unroll(a: String, @unroll b: Boolean = true): // error
+  |            ^
+  |            implementation restriction: Unrolled method cannot be a trait constructor

tests/neg/unroll-traitConstructor.scala

@@ -0,0 +1,8 @@
+//> using options -experimental
+
+import scala.annotation.unroll
+
+trait Unroll(a: String, @unroll b: Boolean = true): // error
+  def show: String = a + b
+
+class Bar(arg: String, bool: Boolean) extends Unroll(arg, bool)