Report rewrite errors at outermost rewrite call

Author: odersky

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

@@ -362,7 +362,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
       if (inlinedMethod == defn.Typelevel_constValue) {
         val constVal = tryConstValue
         if (!constVal.isEmpty) return constVal
-        ctx.error(i"not a constant type: ${callTypeArgs.head}; cannot take constValue")
+        ctx.error(i"not a constant type: ${callTypeArgs.head}; cannot take constValue", call.pos)
       }
       else if (inlinedMethod == defn.Typelevel_constValueOpt) {
         val constVal = tryConstValue
@@ -450,7 +450,14 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
       case (msgArg :: Nil) :: Nil =>
         msgArg.tpe match {
           case ConstantType(Constant(msg: String)) =>
-            ctx.error(msg, call.pos)
+            // Usually `error` is called from within a rewrite method. In this
+            // case we need to report the error at the point of the outermost enclosing inline
+            // call. This way, a defensively written rewrite methid can always
+            // report bad inputs at the point of call instead of revealing its internals.
+            val callToReport = if (enclosingInlineds.nonEmpty) enclosingInlineds.last else call
+            val ctxToReport = ctx.outersIterator.dropWhile(enclosingInlineds(_).nonEmpty).next
+            def issueInCtx(implicit ctx: Context) = ctx.error(msg, callToReport.pos)
+            issueInCtx(ctxToReport)
           case _ =>
         }
       case _ =>