Don't eta expand unary varargs methods

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

@@ -3922,7 +3922,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               else defn.functionArity(ptNorm)
             else
               val nparams = wtp.paramInfos.length
-              if nparams > 0 || pt.eq(AnyFunctionProto) then nparams
+              if nparams > 1
+                  || nparams == 1 && !wtp.isVarArgsMethod
+                  || pt.eq(AnyFunctionProto)
+              then nparams
               else -1 // no eta expansion in this case
           adaptNoArgsUnappliedMethod(wtp, funExpected, arity)
         case _ =>

tests/neg/i16820.check

@@ -0,0 +1,22 @@
+-- Error: tests/neg/i16820.scala:5:11 ----------------------------------------------------------------------------------
+5 |  val x1 = f  // error
+  |           ^
+  |           missing arguments for method f in object Test
+-- [E100] Syntax Error: tests/neg/i16820.scala:6:11 --------------------------------------------------------------------
+6 |  val x2 = g  // error
+  |           ^
+  |           method g in object Test must be called with () argument
+  |
+  | longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i16820.scala:8:14 ----------------------------------------------------------------------------------
+8 |  val x3 = "".formatted // error
+  |           ^^^^^^^^^^^^
+  |           missing arguments for method formatted in class String
+-- Error: tests/neg/i16820.scala:9:40 ----------------------------------------------------------------------------------
+9 |  val x4 = java.nio.file.Paths.get(".").toRealPath // error
+  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |           missing arguments for method toRealPath in trait Path
+-- Error: tests/neg/i16820.scala:13:14 ---------------------------------------------------------------------------------
+13 |def test = Foo(3)  // error
+   |           ^^^^^^
+   |           missing arguments for method apply in object Foo

tests/neg/i16820.scala

@@ -0,0 +1,13 @@
+object Test:
+  def f(xs: Int*) = xs.sum
+  def g() = 1
+
+  val x1 = f  // error
+  val x2 = g  // error
+
+  val x3 = "".formatted // error
+  val x4 = java.nio.file.Paths.get(".").toRealPath // error
+
+// #14567
+case class Foo(x: Int)(xs: String*)
+def test = Foo(3)  // error

tests/pos/i14367.scala

@@ -1,5 +1,5 @@
 def m(i: Int*) = i.sum
-val f1 = m
+val f1: Seq[Int] => Int = m
 val f2 = i => m(i*)
 
 def n(i: Seq[Int]) = i.sum