Replace singleton bounds before doing implicit search

Before searching for implicit arguments, if a the constraint
contains a type parameter
```
   P >: A <: B
```
where `P` occurs in the searched-for type and `A =:= B`, change the constraint
to
```
   P := B
```
instead. This improves the implicit search by making the searched-for type
have fewer uninstantiated type variables.

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

@@ -138,6 +138,31 @@ object Inferencing {
     }
   }
 
+  /** For all type parameters occurring in `tp`:
+   *  If the bounds of `tp` in the current constraint are equal wrt =:=,
+   *  instantiate the type parameter to the lower bound's approximation
+   *  (approximation because of possible F-bounds).
+   */
+  def replaceSingletons(tp: Type)(implicit ctx: Context): Unit = {
+    val tr = new TypeTraverser {
+      def traverse(tp: Type): Unit = {
+        tp match {
+          case param: TypeParamRef =>
+            val constraint = ctx.typerState.constraint
+            if (constraint.contains(param) &&
+                (constraint.fullUpperBound(param) frozen_<:< constraint.fullLowerBound(param))) {
+              typr.println(i"replace singleton $param := ${constraint.fullLowerBound(param)}")
+              ctx.typerState.constraint = constraint.replace(param,
+                ctx.typeComparer.approximation(param, fromBelow = true))
+            }
+          case _ =>
+        }
+        traverseChildren(tp)
+      }
+    }
+    tr.traverse(tp)
+  }
+
   /** If `tree` has a type lambda type, infer its type parameters by comparing with expected type `pt` */
   def inferTypeParams(tree: Tree, pt: Type)(implicit ctx: Context): Tree = tree.tpe match {
     case tl: TypeLambda =>

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

@@ -2587,7 +2587,11 @@ class Typer extends Namer
     def adaptNoArgsImplicitMethod(wtp: MethodType): Tree = {
       assert(wtp.isImplicitMethod)
       val tvarsToInstantiate = tvarsInParams(tree, locked).distinct
-      wtp.paramInfos.foreach(instantiateSelected(_, tvarsToInstantiate))
+      def instantiate(tp: Type): Unit = {
+        instantiateSelected(tp, tvarsToInstantiate)
+        replaceSingletons(tp)
+      }
+      wtp.paramInfos.foreach(instantiate)
       val constr = ctx.typerState.constraint
 
       def dummyArg(tp: Type) = untpd.Ident(nme.???).withTypeUnchecked(tp)