Tweak implicit resolution

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -851,6 +851,9 @@ object Types {
       ctx.typeComparer.isSameType(this, that)
     }
 
+    final def frozen_=:=(that: Type)(implicit ctx: Context): Boolean =
+      ctx.typeComparer.isSameTypeWhenFrozen(this, that)
+
     /** Is this type a primitive value type which can be widened to the primitive value type `that`? */
     def isValueSubType(that: Type)(implicit ctx: Context): Boolean = widen match {
       case self: TypeRef if self.symbol.isPrimitiveValueClass =>

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -67,8 +67,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
     else tp
 
   private def sameBound(lo: Type, hi: Type): Boolean =
-    try ctx.typeComparer.isSameTypeWhenFrozen(lo, hi)
-    catch { case NonFatal(ex) => false }
+    try lo frozen_=:= hi catch { case NonFatal(ex) => false }
 
   private def homogenizeArg(tp: Type) = tp match {
     case TypeBounds(lo, hi) if homogenizedView && sameBound(lo, hi) => homogenize(hi)

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

@@ -1370,11 +1370,14 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     val strippedType2 = stripImplicit(fullType2, +1)
 
     val result = compareWithTypes(strippedType1, strippedType2)
-    if (result != 0) result
-    else if (implicitBalance != 0) -implicitBalance.signum
+    if (result != 0 || !ctx.typerState.test(implicit ctx => strippedType1 =:= strippedType2))
+      result
+    else if (implicitBalance != 0)
+      -implicitBalance.signum
     else if ((strippedType1 `ne` fullType1) || (strippedType2 `ne` fullType2))
       compareWithTypes(fullType1, fullType2)
-    else 0
+    else
+      0
   }}
 
   def narrowMostSpecific(alts: List[TermRef])(implicit ctx: Context): List[TermRef] = track("narrowMostSpecific") {
@@ -1637,7 +1640,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
               //   ps(i) = List(p_i_1, ..., p_i_n)  -- i.e. a column
               // If all p_i_k's are the same, assume the type as formal parameter
               // type of the i'th parameter of the closure.
-              if (isUniform(ps)(ctx.typeComparer.isSameTypeWhenFrozen(_, _))) ps.head
+              if (isUniform(ps)(_ frozen_=:= _)) ps.head
               else WildcardType)
             def isPartial = // we should generate a partial function for the arg
               fn.get.isInstanceOf[untpd.Match] &&

docs/docs/reference/changed-features/implicit-resolution.md

@@ -114,8 +114,10 @@ affect implicits on the language level.
     An alternative A is _more specific_ than an alternative B if
 
      - the relative weight of A over B is greater than the relative weight of B over A, or
-     - the relative weights are the same and A takes more inferable parameters than B, or
-     - the relative weights and the number of inferable parameters are the same and
+     - the relative weights are the same, and the returned types of A and B are
+       unifiable, and A takes more inferable parameters than B, or
+     - the relative weights and the number of inferable parameters are the same, and
+       the returned types of A and B are unifiable, and
        A is more specific than B if all inferable parameters in either alternative are
        replaced by regular parameters.
 

tests/neg/i3430.check

@@ -0,0 +1,2 @@
+<44..44> in i3430.scala
+ambiguous implicit arguments: both object Long in object Ordering and object Short in object Ordering match type Ordering[B] of parameter cmp of method min in trait TraversableOnce

tests/neg/i3430.scala

@@ -1,5 +1,5 @@
 object Test extends App {
 
-  println(Nil.min) // error: no implicit found
+  println(Nil.min) // error: ambiguous
 
 }