Fix #3348: use a new typeState in inferView

`inferImplicit` has an useful invariant that the constraint doesn't
change in the case of implicit resolution failure. However, this
invariant may not hold if completions are triggered during implicit resolution.
By create a new typeState for inferView, we ensure that the useful
invariant holds in such cases.

Author: liufengyun

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

@@ -2224,9 +2224,11 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (isFullyDefined(wtp, force = ForceDegree.all) &&
             ctx.typerState.constraint.ne(prevConstraint)) adapt(tree, pt)
         else err.typeMismatch(tree, pt, failure)
-      if (ctx.mode.is(Mode.ImplicitsEnabled))
-        inferView(tree, pt) match {
+      if (ctx.mode.is(Mode.ImplicitsEnabled)) {
+        val nestedCtx = ctx.fresh.setNewTyperState()
+        inferView(tree, pt)(nestedCtx) match {
           case SearchSuccess(inferred, _, _, _) =>
+            nestedCtx.typerState.commit()
             adapt(inferred, pt)(ctx.retractMode(Mode.ImplicitsEnabled))
           case failure: SearchFailure =>
             if (pt.isInstanceOf[ProtoType] && !failure.isInstanceOf[AmbiguousImplicits])
@@ -2236,6 +2238,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               tree
             else recover(failure)
         }
+      }
       else recover(NoImplicitMatches)
     }