Fix #8250: Allow Expr.betaReduce to drop type ascriptions.

When a blackbox inline def is inlined, a type ascription is added to preserve the apparent return type
of the inline def. Prior to this commit, the quoted.Expr.betaReduce function was unable to properly
inline a known lambda expression coming from an inline def as it would see the type ascription and
give up.

This change instead makes betaReduce drop type ascriptions, allowing these two features to interact properly.

Testing is done by verifying that betaReduce can correctly reduce the application of the result of an inline def
to a constant value, with the reduction result being printed using the `compiletime.code` interpolator.

Author: fhackett

compiler/src/dotty/tools/dotc/tastyreflect/ReflectionCompilerInterface.scala

@@ -2043,6 +2043,8 @@ class ReflectionCompilerInterface(val rootContext: core.Contexts.Context) extend
     val argVals = argVals0.reverse
     val argRefs = argRefs0.reverse
     def rec(fn: Tree): Tree = fn match {
+      case Typed(expr, tpt) =>
+        rec(expr)
       case Inlined(call, bindings, expansion) =>
         // this case must go before closureDef to avoid dropping the inline node
         cpy.Inlined(fn)(call, bindings, rec(expansion))

tests/run-macros/beta-reduce-inline-result.check

@@ -0,0 +1 @@
+4

tests/run-macros/beta-reduce-inline-result/Macro_1.scala

@@ -0,0 +1,10 @@
+import scala.quoted._
+
+object Macros {
+  inline def betaReduce[Arg,Result](inline fn : Arg=>Result)(inline arg: Arg): Result =
+    ${ betaReduceImpl('{ fn })('{ arg }) }
+
+  def betaReduceImpl[Arg,Result](fn: Expr[Arg=>Result])(arg: Expr[Arg])(using qctx : QuoteContext): Expr[Result] =
+    Expr.betaReduce(fn)(arg)
+}
+

tests/run-macros/beta-reduce-inline-result/Test_2.scala

@@ -0,0 +1,12 @@
+import scala.compiletime._
+
+object Test {
+  
+  inline def dummy: Int => Int =
+    (i: Int) => i + 1
+
+  def main(argv : Array[String]) : Unit = {
+    println(code"${Macros.betaReduce(dummy)(3)}")
+  }
+}
+

tests/run-staging/i3876-c.check

@@ -3,8 +3,5 @@
   val f: scala.Function1[scala.Int, scala.Int] {
     def apply(x: scala.Int): scala.Int
   } = ((x: scala.Int) => x.+(x))
-
-  (f: scala.Function1[scala.Int, scala.Int] {
-    def apply(x: scala.Int): scala.Int
-  }).apply(3)
+  f.apply(3)
 }