Add compiletime.ops.string.CharAt

Author: OlivierBlanvillain

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1122,7 +1122,7 @@ class Definitions {
   )
   private val compiletimePackageBooleanTypes: Set[Name] = Set(tpnme.Not, tpnme.Xor, tpnme.And, tpnme.Or)
   private val compiletimePackageStringTypes: Set[Name] = Set(
-    tpnme.Plus, tpnme.Length, tpnme.Substring, tpnme.Matches
+    tpnme.Plus, tpnme.Length, tpnme.Substring, tpnme.Matches, tpnme.CharAt
   )
   private val compiletimePackageOpTypes: Set[Name] =
     Set(tpnme.S)

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -235,6 +235,7 @@ object StdNames {
     final val Plus: N                 = "+"
     final val S: N                    = "S"
     final val Substring: N            = "Substring"
+    final val CharAt: N               = "CharAt"
     final val Times: N                = "*"
     final val ToInt: N                = "ToInt"
     final val ToLong: N               = "ToLong"

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4423,6 +4423,8 @@ object Types {
               case tpnme.Matches    => constantFold2(stringValue, _ matches _)
               case tpnme.Substring  =>
                 constantFold3(stringValue, intValue, intValue, (s, b, e) => s.substring(b, e))
+              case tpnme.CharAt     =>
+                constantFold2AB(stringValue, intValue, _ charAt _)
               case _ => None
             } else if (owner == defn.CompiletimeOpsBooleanModuleClass) name match {
               case tpnme.Not        => constantFold1(boolValue, x => !x)

library/src/scala/compiletime/ops/string.scala

@@ -43,3 +43,14 @@ object string:
    */
   @experimental
   type Matches[S <: String, Regex <: String] <: Boolean
+
+  /** Returns the Char type at the specified index.
+   *  An index ranges from 0 to Length[S] - 1. The first Char of
+   *  the sequence is at index 0, the next at index 1, and so on.
+   *  ```scala
+   *  val c: CharAt["hello", 0] = 'h'
+   *  ```
+   *  @syntax markdown
+   */
+  @experimental
+  type CharAt[S <: String, Idx <: Int] <: Char

tests/neg/singleton-ops-string.scala

@@ -15,4 +15,23 @@ object Test {
 
   val t9: Matches["hamburger", "ham.*"] = true
   val t10: Matches["hamburger", "ham.*"] = false // error
+
+  val t11: CharAt["String", 0] = 'S'
+  val t12: CharAt["String", 1] = 't'
+  val t13: CharAt["String", 2] = '!' // error
+  //                             ^^^
+  //                             Found:    ('!' : Char)
+  //                             Required: ('r' : Char)
+  val t14: CharAt["String", 3] = '!' // error
+  //                             ^^^
+  //                             Found:    ('!' : Char)
+  //                             Required: ('i' : Char)
+  val t15: CharAt["String", 4] = 'n'
+  val t16: CharAt["String", 5] = 'g'
+  val t17: CharAt["String", 6]  = '!' // error
+  //       ^
+  //       String index out of range: 6
+  val t18: CharAt["String", -1] = '?' // error
+  //       ^
+  //       String index out of range: -1
 }