Add reflect defn.{FunctionClass,ContextFunctionClass}

The old `FunctionClass` will need to be deprecated as we will remove
`ErasedFunctionN` and `ErasedContextFunctionN`. We will replace this API
with a simpler version that can return `FunctionN` or `ContextFunctionN`,
the only two stable function classes we have in the compiler/TASTy.

Other new function classes will be encoded with the more general refined
function type encoding, generalization of the `PolyFunction` encoding.
This implies that we won't need to add other kind of function classes to
the reflect API.

Part of the fix for #16847

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -2768,7 +2768,16 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
       def SomeModule: Symbol = dotc.core.Symbols.defn.SomeClass.companionModule
       def ProductClass: Symbol = dotc.core.Symbols.defn.ProductClass
       def FunctionClass(arity: Int, isImplicit: Boolean = false, isErased: Boolean = false): Symbol =
+        if arity < 0 then throw IllegalArgumentException(s"arity: $arity")
         dotc.core.Symbols.defn.FunctionSymbol(arity, isImplicit, isErased)
+      def FunctionClass(arity: Int): Symbol =
+        FunctionClass(arity, false, false)
+      def FunctionClass(arity: Int, isContextual: Boolean): Symbol =
+        FunctionClass(arity, isContextual, false)
+
+      def ContextFunctionClass(arity: Int): Symbol =
+        if arity < 0 then throw IllegalArgumentException(s"arity: $arity")
+        dotc.core.Symbols.defn.FunctionSymbol(arity, true, false)
       def TupleClass(arity: Int): Symbol =
         dotc.core.Symbols.defn.TupleType(arity).nn.classSymbol.asClass
       def isTupleClass(sym: Symbol): Boolean =

library/src/scala/quoted/Quotes.scala

@@ -4254,8 +4254,27 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
       *   -  ...
       *   -  Nth element is `FunctionN`
       */
+      // TODO: deprecate in 3.4 and stabilize FunctionClass(Int)/ContextFunctionClass(Int)
+      // @deprecated("Use overload of `FunctionClass` with 1 or 2 arguments","3.4")
       def FunctionClass(arity: Int, isImplicit: Boolean = false, isErased: Boolean = false): Symbol
 
+      /** Class symbol of a function class `scala.FunctionN`.
+       *
+       *  @param arity the arity of the function where `0 <= arity`
+       *  @return class symbol of `scala.FunctionN` where `N == arity`
+       */
+      @experimental
+      def FunctionClass(arity: Int): Symbol
+
+      /** Class symbol of a context function class `scala.ContextFunctionN`.
+       *
+       *  @param arity the arity of the function where `0 <= arity`
+       *  @param isContextual if it is a `scala.ContextFunctionN`
+       *  @return class symbol of `scala.FunctionN` or `scala.ContextFunctionN` where `N == arity`
+       */
+      @experimental
+      def FunctionClass(arity: Int, isContextual: Boolean): Symbol
+
       /** Function-like object that maps arity to symbols for classes `scala.TupleX`.
       *   -  0th element is `NoSymbol`
       *   -  1st element is `NoSymbol`

tests/run-custom-args/tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -66,7 +66,9 @@ val experimentalDefinitionInLibrary = Set(
   "scala.annotation.MacroAnnotation",
 
    //// New APIs: Quotes
-  // Can be stabilized in 3.3.0 (unsure) or later
+  // Should be stabilized in 3.4.0
+  "scala.quoted.Quotes.reflectModule.defnModule.FunctionClass",
+  // Can be stabilized in 3.4.0 (unsure) or later
   "scala.quoted.Quotes.reflectModule.CompilationInfoModule.XmacroSettings",
   "scala.quoted.Quotes.reflectModule.FlagsModule.JavaAnnotation",
   // Cant be stabilized yet.

tests/run-macros/tasty-definitions-1.check

@@ -57,31 +57,57 @@ Function23
 Function24
 Function25
 ContextFunction0
+ContextFunction0
+ContextFunction1
 ContextFunction1
 ContextFunction2
+ContextFunction2
+ContextFunction3
 ContextFunction3
 ContextFunction4
+ContextFunction4
+ContextFunction5
 ContextFunction5
 ContextFunction6
+ContextFunction6
 ContextFunction7
+ContextFunction7
+ContextFunction8
 ContextFunction8
 ContextFunction9
+ContextFunction9
 ContextFunction10
+ContextFunction10
+ContextFunction11
 ContextFunction11
 ContextFunction12
+ContextFunction12
 ContextFunction13
+ContextFunction13
+ContextFunction14
 ContextFunction14
 ContextFunction15
+ContextFunction15
+ContextFunction16
 ContextFunction16
 ContextFunction17
+ContextFunction17
+ContextFunction18
 ContextFunction18
 ContextFunction19
+ContextFunction19
 ContextFunction20
+ContextFunction20
+ContextFunction21
 ContextFunction21
 ContextFunction22
+ContextFunction22
 ContextFunction23
+ContextFunction23
+ContextFunction24
 ContextFunction24
 ContextFunction25
+ContextFunction25
 ErasedFunction1
 ErasedFunction2
 ErasedFunction3

tests/run-macros/tasty-definitions-1/quoted_1.scala

@@ -60,6 +60,7 @@ object Macros {
       printout(defn.FunctionClass(i).name)
 
     for (i <- 0 to 25)
+      printout(defn.FunctionClass(i, isContextual = true).name)
       printout(defn.FunctionClass(i, isImplicit = true).name)
 
     for (i <- 1 to 25)