Avoid nonsensical error message in processByNameArgs

Transforming the argument can already cause errors coming from
TypeAssigne rthat need to be caught instead of being issued.

Author: odersky

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

@@ -89,7 +89,6 @@ object ErrorReporting {
       if (tree.tpe.widen.exists)
         i"${exprStr(tree)} does not take ${kind}parameters"
       else {
-        if (ctx.settings.Ydebug.value) new FatalError("").printStackTrace()
         i"undefined: $tree # ${tree.uniqueId}: ${tree.tpe.toString} at ${ctx.phase}"
       }
 

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

@@ -464,7 +464,8 @@ object Nullables with
    */
   def postProcessByNameArgs(fn: TermRef, app: Tree)(given ctx: Context): Tree =
     fn.widen match
-      case mt: MethodType if mt.paramInfos.exists(_.isInstanceOf[ExprType]) =>
+      case mt: MethodType
+      if mt.paramInfos.exists(_.isInstanceOf[ExprType]) && !fn.symbol.is(Inline) =>
         app match
           case Apply(fn, args) =>
             val dropNotNull = new TreeMap with
@@ -487,10 +488,10 @@ object Nullables with
                 case _ => super.typedUnadapted(t, pt, locked)
 
             def postProcess(formal: Type, arg: Tree): Tree =
-              val arg1 = dropNotNull.transform(arg)
+              val nestedCtx = ctx.fresh.setNewTyperState()
+              val arg1 = dropNotNull.transform(arg)(given nestedCtx)
               if arg1 eq arg then arg
               else
-                val nestedCtx = ctx.fresh.setNewTyperState()
                 val arg2 = retyper.typed(arg1, formal)(given nestedCtx)
                 if nestedCtx.reporter.hasErrors || !(arg2.tpe <:< formal) then
                   ctx.error(em"""This argument was typed using flow assumptions about mutable variables

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

@@ -411,6 +411,7 @@ trait TypeAssigner {
         else
           errorType(i"wrong number of arguments at ${ctx.phase.prev} for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.sourcePos)
       case t =>
+        if (ctx.settings.Ydebug.value) new FatalError("").printStackTrace()
         errorType(err.takesNoParamsStr(fn, ""), tree.sourcePos)
     }
     ConstFold(tree.withType(ownType))

tests/neg-custom-args/explicit-nulls/byname-nullables1.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/explicit-nulls/byname-nullables1.scala:9:22 --------------------------------------------
+9 |  if x != null then f(x.fld != null) // error
+  |                      ^^^^^^^^^^^^^
+  |                      This argument was typed using flow assumptions about mutable variables
+  |                      but it is passed to a by-name parameter where such flow assumptions are unsound.
+  |                      Wrapping the argument in `byName(...)` fixes the problem by disabling the flow assumptions.
+  |
+  |                      `byName` needs to be imported from the `scala.compiletime` package.

tests/neg-custom-args/explicit-nulls/byname-nullables1.scala

@@ -0,0 +1,9 @@
+def f(op: => Boolean): Unit = ()
+def f(op: Int): Unit = ()
+
+class C with
+  var fld: String | Null = null
+
+def test() =
+  var x: C | Null = C()
+  if x != null then f(x.fld != null) // error