Make sure we don't lose erased in method types on Setup

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -523,13 +523,16 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
               // substitute `x.f.type`, `x` becomes a `TermParamRef`. But the new method
               // type is still under initialization and `paramInfos` is still `null`,
               // so the new `NamedType` will not have a denoation.
+              def adaptedInfo(psym: Symbol, info: mt.PInfo): mt.PInfo = mt.companion match
+                case mtc: MethodTypeCompanion => mtc.adaptParamInfo(psym, info).asInstanceOf[mt.PInfo]
+                case _ => info
               mt.companion(mt.paramNames)(
                 mt1 =>
                   if !paramSignatureChanges && !mt.isParamDependent && prevLambdas.isEmpty then
                     mt.paramInfos
                   else
                     val subst = SubstParams(psyms :: prevPsymss, mt1 :: prevLambdas)
-                    psyms.map(psym => subst(psym.nextInfo).asInstanceOf[mt.PInfo]),
+                    psyms.map(psym => adaptedInfo(psym, subst(psym.nextInfo).asInstanceOf[mt.PInfo])),
                 mt1 =>
                   integrateRT(mt.resType, psymss.tail, resType, psyms :: prevPsymss, mt1 :: prevLambdas)
               )

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4178,24 +4178,28 @@ object Types extends TypeUtils {
      *   - wrap types of parameters that have an @allowConversions annotation with Into[_]
      */
     def fromSymbols(params: List[Symbol], resultType: Type)(using Context): MethodType =
+      apply(params.map(_.name.asTermName))(
+         tl => params.map(p => tl.integrate(params, adaptParamInfo(p))),
+         tl => tl.integrate(params, resultType))
+
+    /** Adapt info of parameter symbol to be integhrated into corresponding MethodType
+     *  using the scheme described in `fromSymbols`.
+     */
+    def adaptParamInfo(param: Symbol, pinfo: Type)(using Context): Type =
       def addAnnotation(tp: Type, cls: ClassSymbol, param: Symbol): Type = tp match
         case ExprType(resType) => ExprType(addAnnotation(resType, cls, param))
         case _ => AnnotatedType(tp, Annotation(cls, param.span))
-
-      def paramInfo(param: Symbol) =
-        var paramType = param.info
-          .annotatedToRepeated
-          .mapIntoAnnot(defn.IntoAnnot, defn.IntoParamAnnot)
-        if param.is(Inline) then
-          paramType = addAnnotation(paramType, defn.InlineParamAnnot, param)
-        if param.is(Erased) then
-          paramType = addAnnotation(paramType, defn.ErasedParamAnnot, param)
-        paramType
-
-      apply(params.map(_.name.asTermName))(
-         tl => params.map(p => tl.integrate(params, paramInfo(p))),
-         tl => tl.integrate(params, resultType))
-    end fromSymbols
+      var paramType = pinfo
+        .annotatedToRepeated
+        .mapIntoAnnot(defn.IntoAnnot, defn.IntoParamAnnot)
+      if param.is(Inline) then
+        paramType = addAnnotation(paramType, defn.InlineParamAnnot, param)
+      if param.is(Erased) then
+        paramType = addAnnotation(paramType, defn.ErasedParamAnnot, param)
+      paramType
+
+    def adaptParamInfo(param: Symbol)(using Context): Type =
+      adaptParamInfo(param, param.info)
 
     def apply(paramNames: List[TermName])(paramInfosExp: MethodType => List[Type], resultTypeExp: MethodType => Type)(using Context): MethodType =
       checkValid(unique(new CachedMethodType(paramNames)(paramInfosExp, resultTypeExp, self)))

Diff for: tests/pos-custom-args/captures/erased-methods.scala

@@ -0,0 +1,20 @@
+import language.experimental.saferExceptions
+import language.experimental.erasedDefinitions
+import language.experimental.captureChecking
+
+class Ex1 extends Exception("Ex1")
+class Ex2 extends Exception("Ex2")
+class Ex3 extends Exception("Ex3")
+
+def foo8a(i: Int) =
+  (erased xx1: CanThrow[Ex2]^) ?=> throw new Ex2
+
+def foo9a(i: Int)
+  : (erased x$0: CanThrow[Ex3]^)
+    ?=> (erased x$1: CanThrow[Ex2]^)
+    ?=> (erased x$2: CanThrow[Ex1]^)
+    ?=> Unit
+  = (erased x$1: CanThrow[Ex3]^)
+     ?=> (erased x$2: CanThrow[Ex2]^)
+     ?=> (erased x$3: CanThrow[Ex1]^)
+     ?=> throw new Ex3