Fix #18763: Run type inference before implicit search

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

@@ -27,8 +27,8 @@ object Inferencing {
    *  but only if the overall result of `isFullyDefined` is `true`.
    *  Variables that are successfully minimized do not count as uninstantiated.
    */
-  def isFullyDefined(tp: Type, force: ForceDegree.Value)(using Context): Boolean =
-    withFreshTyperState(new IsFullyDefinedAccumulator(force).process(tp), x => x)
+  def isFullyDefined(tp: Type, force: ForceDegree.Value, ifProto: Boolean = false)(using Context): Boolean =
+    withFreshTyperState(new IsFullyDefinedAccumulator(force, ifProto = ifProto).process(tp), x => x)
 
   /** Try to fully define `tp`. Return whether constraint has changed.
    *  Any changed constraint is kept.
@@ -161,7 +161,7 @@ object Inferencing {
    *  Instance types can be improved by replacing covariant occurrences of Nothing
    *  with fresh type variables, if `force` allows this in its `canImprove` implementation.
    */
-  private class IsFullyDefinedAccumulator(force: ForceDegree.Value, minimizeSelected: Boolean = false)
+  private class IsFullyDefinedAccumulator(force: ForceDegree.Value, minimizeSelected: Boolean = false, ifProto: Boolean = false)
     (using Context) extends TypeAccumulator[Boolean] {
 
     /** Replace toplevel-covariant occurrences (i.e. covariant without double flips)
@@ -233,8 +233,10 @@ object Inferencing {
       val tpd = tp.dealias
       if tpd ne tp then apply(x, tpd)
       else tp match
-        case _: WildcardType | _: ProtoType =>
+        case _: WildcardType =>
           false
+        case tp: ProtoType =>
+          ifProto && foldOver(x, tp)
         case tvar: TypeVar if !tvar.isInstantiated =>
           force.appliesTo(tvar)
           && ctx.typerState.constraint.contains(tvar)

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

@@ -396,7 +396,7 @@ object ProtoTypes {
         args: List[untpd.Tree] = this.args,
         resultType: Type = this.resultType,
         typer: Typer = this.typer,
-        constrainResultDeep: Boolean = this.constrainResultDeep): FunProto =
+        constrainResultDeep: Boolean = this.constrainResultDeep)(using Context): FunProto =
       if (args eq this.args)
           && (resultType eq this.resultType)
           && (typer eq this.typer)

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

@@ -54,6 +54,7 @@ import transform.CheckUnused.OriginalName
 import scala.annotation.{unchecked as _, *}
 import dotty.tools.dotc.util.chaining.*
 import dotty.tools.dotc.ast.untpd.Mod
+import dotty.tools.dotc.reporting.Reporter.NoReporter
 
 object Typer {
 
@@ -4357,6 +4358,27 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 else formals1
               implicitArgs(formals2, argIndex + 1, pt)
 
+
+            def doesntContainsWildcards = {
+              val newCtx = ctx.fresh.setNewScope.setReporter(new reporting.ThrowingReporter(NoReporter))
+              val pt1 = pt.deepenProtoTrans(using newCtx)
+              try {
+                isFullyDefined(pt1, ForceDegree.none, ifProto = true)(using newCtx)
+              } catch {
+                case _: UnhandledError => false
+              }
+            }
+            val pt1 = pt.deepenProtoTrans
+            if (pt1 `ne` pt)
+              && (pt1 ne sharpenedPt)
+              && formal.typeSymbol != defn.ClassTagClass
+              && !isFullyDefined(formal, ForceDegree.none)
+              && !formal.existsPart(ty => {
+                  val dty = ty.dealias
+                  (dty ne ty) && ty.isInstanceOf[TypeVar] && dty.isInstanceOf[TypeVar]
+                }, StopAt.Static, forceLazy = false)
+              && doesntContainsWildcards then
+              constrainResult(tree.symbol, wtp, wildApprox(pt1))
             val arg = inferImplicitArg(formal, tree.span.endPos)
 
             def canProfitFromMoreConstraints =
@@ -4367,7 +4389,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
             arg.tpe match
               case failed: SearchFailureType if canProfitFromMoreConstraints =>
-                val pt1 = pt.deepenProtoTrans
                 if (pt1 `ne` pt) && (pt1 ne sharpenedPt) && constrainResult(tree.symbol, wtp, pt1)
                 then return implicitArgs(formals, argIndex, pt1)
               case _ =>