Fix #6368: Check that secondary constructors call a preceding constru…

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

@@ -1557,10 +1557,20 @@ class Typer extends Namer
       PrepareInlineable.registerInlineInfo(sym, _ => rhs1)
     }
 
-    if (sym.isConstructor && !sym.isPrimaryConstructor)
+    if (sym.isConstructor && !sym.isPrimaryConstructor) {
       for (param <- tparams1 ::: vparamss1.flatten)
         checkRefsLegal(param, sym.owner, (name, sym) => sym.is(TypeParam), "secondary constructor")
 
+      def checkThisConstrCall(tree: Tree): Unit = tree match {
+        case app: Apply if untpd.isSelfConstrCall(app) =>
+          if (sym.span.exists && app.symbol.span.exists && sym.span.start <= app.symbol.span.start)
+            ctx.error("secondary constructor must call a preceding constructor", app.sourcePos)
+        case Block(call :: _, _) => checkThisConstrCall(call)
+        case _ =>
+      }
+      checkThisConstrCall(rhs1)
+    }
+
     assignType(cpy.DefDef(ddef)(name, tparams1, vparamss1, tpt1, rhs1), sym).setDefTree
       //todo: make sure dependent method types do not depend on implicits or by-name params
   }

tests/neg/i6368.scala

@@ -0,0 +1,15 @@
+class A(x: Int, y: Int) {
+  def this() = {
+    this()   // error
+  }
+
+  def this(x: Long) = {
+    this(x.toInt)  // error
+  }
+
+  def this(a: Int) = {
+    this(a, a)  // OK
+    val b: Int = c
+    val c = 2
+  }
+}