Don't lock in failed callee type typings

When we reach out to get the callee type, we might get an error, e.g.
if the callee is overloaded. Example:
```
ch => sb.append(ch)
```
Here we might want to call calleeType to find the argument type
of `sb.append` in order to use this type as the type of `ch`. But
this fails since `append` is overloaded on StringBuffer.

Prevously this did not matter since we called calleeType only as a last
effort, so if it failed the whole typing failed. But with the changes
forseen for fiing #8111 we need to be able to compute calleeType and
retract it if it does not work.

Author: odersky

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

@@ -1013,11 +1013,16 @@ class Typer extends Namer
             case untpd.TypedSplice(expr1) =>
               expr1.tpe
             case _ =>
+              given nestedCtx as Context = ctx.fresh.setNewTyperState()
               val protoArgs = args map (_ withType WildcardType)
               val callProto = FunProto(protoArgs, WildcardType)(this, app.isGivenApply)
               val expr1 = typedExpr(expr, callProto)
-              fnBody = cpy.Apply(fnBody)(untpd.TypedSplice(expr1), args)
-              expr1.tpe
+              if nestedCtx.reporter.hasErrors then NoType
+              else
+                given Context = ctx
+                nestedCtx.typerState.commit()
+                fnBody = cpy.Apply(fnBody)(untpd.TypedSplice(expr1), args)
+                expr1.tpe
           }
         else NoType
       case _ =>