Infer parameters of eta applications with vararg parameters

Fixes scala#14367

Author: odersky

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

@@ -1227,8 +1227,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case mtpe: MethodType =>
         val pos = paramIndex(param.name)
         if pos < mtpe.paramInfos.length then
-          val ptype = mtpe.paramInfos(pos)
-          if ptype.isRepeatedParam then NoType else ptype
+          mtpe.paramInfos(pos)
+            // This works only if vararg annotations match up.
+            // See neg/i14367.scala for an example where the inferred type is mispredicated. 
+            // Nevertheless, the alternative would be to give up completely, so this is
+            // defensible.
         else NoType
       case _ => NoType
     if target.exists then formal <:< target
@@ -1317,10 +1320,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
      */
     var fnBody = tree.body
 
-    def refersTo(arg: untpd.Tree, param: untpd.ValDef): Boolean = arg match {
+    def refersTo(arg: untpd.Tree, param: untpd.ValDef): Boolean = arg match
       case Ident(name) => name == param.name
+      case Typed(arg1, _) if untpd.isWildcardStarArg(arg) => refersTo(arg1, param)
       case _ => false
-    }
 
     /** If parameter `param` appears exactly once as an argument in `args`,
      *  the singleton list consisting of its position in `args`, otherwise `Nil`.

tests/neg/i14367.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i14367.scala:2:16 -------------------------------------------------------------
+2 |val h2 = i => p(i)  // error: Found (i : Seq[Int]), Required: Int
+  |                ^
+  |                Found:    (i : Seq[Int])
+  |                Required: Int
+
+longer explanation available when compiling with `-explain`

tests/neg/i14367.scala

@@ -0,0 +1,5 @@
+def p(i: Int*) = i.sum
+val h2 = i => p(i)  // error: Found (i : Seq[Int]), Required: Int
+  // It would be more logical to fail with a "missing parameter type", however.
+
+

tests/pos/i14367.scala

@@ -0,0 +1,7 @@
+def m(i: Int*) = i.sum
+val f1 = m
+val f2 = i => m(i*)
+
+def n(i: Seq[Int]) = i.sum
+val g1 = n
+val g2 = i => n(i)