Strengthen requirement for auto-tupling

Was: corresponding parameter types "are compatible".
Now: corresponding parameter types "conform".

This avoids the inconsistency mentioned by @retronym in #897.

Author: odersky

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

@@ -618,7 +618,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         formal.derivesFrom(pclass) &&
         formal.baseArgTypes(pclass).corresponds(params) {
           (argType, param) =>
-            param.tpt.isEmpty || isCompatible(argType, typedAheadType(param.tpt).tpe)
+            param.tpt.isEmpty || argType <:< typedAheadType(param.tpt).tpe
         }
       }
 

test/dotc/tests.scala

@@ -111,7 +111,7 @@ class tests extends CompilerTest {
   @Test def neg_abstractOverride() = compileFile(negDir, "abstract-override", xerrors = 2)
   @Test def neg_blockescapes() = compileFile(negDir, "blockescapesNeg", xerrors = 1)
   @Test def neg_bounds() = compileFile(negDir, "bounds", xerrors = 2)
-  @Test def neg_functionArity() = compileFile(negDir, "function-arity", xerrors = 5)
+  @Test def neg_functionArity() = compileFile(negDir, "function-arity", xerrors = 7)
   @Test def neg_typedapply() = compileFile(negDir, "typedapply", xerrors = 3)
   @Test def neg_typedIdents() = compileDir(negDir, "typedIdents", xerrors = 2)
   @Test def neg_assignments() = compileFile(negDir, "assignments", xerrors = 3)

tests/neg/function-arity.scala

@@ -22,3 +22,7 @@ object Test {
   def foo(a: Any => String) = ()
   foo((a: Int, b: String) => a + b) // error: none of the overloaded alternatives of method foo match arguments (Int, Int)
 }
+object jasonComment {
+  implicit def i2s(i: Int): String = i.toString
+  ((x: String, y: String) => 42) : (((Int, Int)) => String) // error
+}