Optimization: Eliminate a hot closure in Implicits

Author: odersky

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

@@ -92,23 +92,26 @@ object Implicits {
 
         def viewCandidateKind(tpw: Type, argType: Type, resType: Type): Candidate.Kind = {
 
-          def methodCandidateKind(mt: MethodType, formal: => Type) =
+          def methodCandidateKind(mt: MethodType, approx: Boolean) =
             if (!mt.isImplicitMethod &&
-                mt.paramInfos.lengthCompare(1) == 0 &&
-                ctx.test(implicit ctx => argType relaxed_<:< formal))
+                mt.paramInfos.lengthCompare(1) == 0 && {
+                  var formal = widenSingleton(mt.paramInfos.head)
+                  if (approx) formal = wildApprox(formal)
+                  ctx.test(implicit ctx => argType relaxed_<:< formal)
+                })
               Candidate.Conversion
             else
               Candidate.None
 
           tpw match {
             case mt: MethodType =>
-              methodCandidateKind(mt, widenSingleton(mt.paramInfos.head))
+              methodCandidateKind(mt, approx = false)
             case poly: PolyType =>
               // We do not need to call ProtoTypes#constrained on `poly` because
               // `candidateKind` is always called with mode TypevarsMissContext enabled.
               poly.resultType match {
                 case mt: MethodType =>
-                  methodCandidateKind(mt, wildApprox(widenSingleton(mt.paramInfos.head)))
+                  methodCandidateKind(mt, approx = true)
                 case rtp =>
                   viewCandidateKind(wildApprox(rtp), argType, resType)
               }