Fix for regression with inference at arity 21+.

A classic "off by two" error.  Closes SI-4545, SI-5633.

Author: paulp

src/compiler/scala/reflect/internal/Definitions.scala

@@ -601,8 +601,8 @@ trait Definitions extends reflect.api.StandardDefinitions {
 
     def isFunctionType(tp: Type): Boolean = tp.normalize match {
       case TypeRef(_, sym, args) if args.nonEmpty =>
-        val len = args.length
-        len < MaxFunctionArity && sym == FunctionClass(len - 1)
+        val arity = args.length - 1   // -1 is the return type
+        arity <= MaxFunctionArity && sym == FunctionClass(arity)
       case _ =>
         false
     }

test/files/pos/t4545.scala

@@ -0,0 +1,14 @@
+object Test {
+  def f[A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T](table: Tuple20[A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T])(fun: (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T) => Unit) {
+  }
+  def g[A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U](table: Tuple21[A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U])(fun: (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U) => Unit) {
+  }
+
+  def g20 = f(
+    (  1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1)
+  ) { case ((a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t)) => () }
+
+  def g21 = g(
+      (1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1)
+  ) { case ((a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u)) => () }
+}