fix #7722 prevent alpha annot on constructor

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

@@ -1643,6 +1643,10 @@ class Typer extends Namer
       PrepareInlineable.registerInlineInfo(sym, _ => rhs1)
 
     if (sym.isConstructor && !sym.isPrimaryConstructor) {
+      val ename = sym.erasedName
+      if (ename != sym.name)
+        ctx.error(em"@alpha annotation ${'"'}$ename${'"'} may not be used on a constructor", ddef.sourcePos)
+
       for (param <- tparams1 ::: vparamss1.flatten)
         checkRefsLegal(param, sym.owner, (name, sym) => sym.is(TypeParam), "secondary constructor")
 

tests/neg/7722.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/7722.scala:2:35 ------------------------------------------------------------------------------------
+2 |  @scala.annotation.alpha("E") def this() = this(3) // error
+  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |  @alpha annotation "E" may not be used on a constructor

tests/neg/7722.scala

@@ -0,0 +1,8 @@
+class A(i:Int){
+  @scala.annotation.alpha("E") def this() = this(3) // error
+}
+object O{
+  def main(args: Array[String]) = {
+    new A()
+  }
+}