Add comments and better error messages in case of compiler bug

Author: nicolasstucki

library/src/scala/compiletime/package.scala

@@ -28,6 +28,11 @@ package object compiletime {
    *  ```scala
    *  error(code"My error of this code: ${println("foo")}")
    *  ```
+   *  or
+   *  ```scala
+   *  inline def errorOnThisCode(inline x: Any) =
+   *    error(code"My error of this code: $x")
+   *  ```
    */
   inline def error(inline msg: String): Nothing = ???
 
@@ -50,7 +55,9 @@ package object compiletime {
     * @note only `inline` arguments will be displayed as "code".
     *       Other values may display unintutively.
     */
-    transparent inline def code (inline args: Any*): String = ???
+    transparent inline def code (inline args: Any*): String =
+      // implemented in dotty.tools.dotc.typer.Inliner.Intrinsics
+      error("`code` was not evaluated by the compiler")
 
   end extension
 
@@ -68,18 +75,24 @@ package object compiletime {
    *  twice(m) // error: expected a constant value but found: m
    *  ```
    */
-  inline def requireConst(inline x: Boolean | Byte | Short | Int | Long | Float | Double | Char | String): Unit = ()
+  inline def requireConst(inline x: Boolean | Byte | Short | Int | Long | Float | Double | Char | String): Unit =
+    // implemented in dotty.tools.dotc.typer.Inliner
+    error("`requireConst` was not evaluated by the compiler")
 
   /** Same as `constValue` but returns a `None` if a constant value
    *  cannot be constructed from the provided type. Otherwise returns
    *  that value wrapped in `Some`.
    */
-  inline def constValueOpt[T]: Option[T] = ???
+  inline def constValueOpt[T]: Option[T] =
+    // implemented in dotty.tools.dotc.typer.Inliner
+    error("`constValueOpt` was not evaluated by the compiler")
 
   /** Given a constant, singleton type `T`, convert it to a value
    *  of the same singleton type. For example: `assert(constValue[1] == 1)`.
    */
-  inline def constValue[T]: T = ???
+  inline def constValue[T]: T =
+    // implemented in dotty.tools.dotc.typer.Inliner
+    error("`constValue` was not evaluated by the compiler")
 
   /** Given a tuple type `(X1, ..., Xn)`, returns a tuple value
    *  `(constValue[X1], ..., constValue[Xn])`.
@@ -106,8 +119,8 @@ package object compiletime {
    *
    *  the returned value would be `2`.
    */
-  transparent inline def summonFrom[T](f: Nothing => T): T = ???
-
+  transparent inline def summonFrom[T](f: Nothing => T): T =
+    error("`summonFrom` was not evaluated by the compiler")
 
   /** Summon a given value of type `T`. Usually, the argument is not passed explicitly.
    *  The summoning is delayed until the call has been fully inlined.

library/src/scala/compiletime/testing/package.scala

@@ -11,6 +11,7 @@ package object testing {
    *  The code should be a sequence of expressions or statements that may appear in a block.
    */
   inline def typeChecks(inline code: String): Boolean =
+    // implemented in package dotty.tools.dotc.typer.Inliner.Intrinsics
     error("`typeChecks` was not checked by the compiler")
 
   /** Whether the code type checks in the current context? If not,
@@ -27,5 +28,6 @@ package object testing {
    *  The code should be a sequence of expressions or statements that may appear in a block.
    */
   inline def typeCheckErrors(inline code: String): List[Error] =
+    // implemented in package dotty.tools.dotc.typer.Inliner.Intrinsics
     error("`typeCheckErrors` was not checked by the compiler")
 }