Disallow by-name erased parameters

Author: natsukagami

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

@@ -15,6 +15,7 @@ import MegaPhase.*
 import Decorators.*
 import typer.RefChecks
 import reporting.trace
+import dotty.tools.dotc.core.Names.Name
 
 /** This phase implements the following transformations:
  *
@@ -79,11 +80,14 @@ class ElimByName extends MiniPhase, InfoTransformer:
     case ExprType(rt) if exprBecomesFunction(sym) =>
       defn.ByNameFunction(rt)
     case tp: MethodType =>
-      def exprToFun(tp: Type) = tp match
-        case ExprType(rt) => defn.ByNameFunction(rt)
+      def exprToFun(tp: Type, name: Name) = tp match
+        case ExprType(rt) =>
+          if rt.hasAnnotation(defn.ErasedParamAnnot) then
+            report.error(em"By-name parameter cannot be erased: $name", sym.srcPos)
+          defn.ByNameFunction(rt)
         case tp => tp
       tp.derivedLambdaType(
-        paramInfos = tp.paramInfos.mapConserve(exprToFun),
+        paramInfos = tp.paramInfos.zipWithConserve(tp.paramNames)(exprToFun),
         resType = transformInfo(tp.resType, sym))
     case tp: PolyType =>
       tp.derivedLambdaType(resType = transformInfo(tp.resType, sym))

tests/neg-custom-args/erased/by-name.scala

@@ -0,0 +1,4 @@
+def f(x: => Int, erased y: => Int) = x // error
+def g(erased x: => Int, y: => Int) = y // error
+
+val h: (erased => Int, Int) => Int = (erased x, y) => y // error