Special-case summonInline to yield implicitNotFound errors

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

@@ -233,6 +233,7 @@ class Definitions {
     @tu lazy val Compiletime_constValue   : Symbol = CompiletimePackageClass.requiredMethod("constValue")
     @tu lazy val Compiletime_constValueOpt: Symbol = CompiletimePackageClass.requiredMethod("constValueOpt")
     @tu lazy val Compiletime_summonFrom   : Symbol = CompiletimePackageClass.requiredMethod("summonFrom")
+    @tu lazy val Compiletime_summonInline   : Symbol = CompiletimePackageClass.requiredMethod("summonInline")
   @tu lazy val CompiletimeTestingPackage: Symbol = requiredPackage("scala.compiletime.testing")
     @tu lazy val CompiletimeTesting_typeChecks: Symbol = CompiletimeTestingPackage.requiredMethod("typeChecks")
     @tu lazy val CompiletimeTesting_typeCheckErrors: Symbol = CompiletimeTestingPackage.requiredMethod("typeCheckErrors")

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

@@ -701,7 +701,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
           return Intrinsics.codeOf(arg, call.srcPos)
       case _ =>
 
-  	// Special handling of `constValue[T]` and `constValueOpt[T]`
+  	// Special handling of `constValue[T]`, `constValueOpt[T], and summonInline[T]`
     if (callTypeArgs.length == 1)
       if (inlinedMethod == defn.Compiletime_constValue) {
         val constVal = tryConstValue
@@ -718,6 +718,19 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
           else New(defn.SomeClass.typeRef.appliedTo(constVal.tpe), constVal :: Nil)
         )
       }
+      else if (inlinedMethod == defn.Compiletime_summonInline) {
+        def searchImplicit(tpt: Tree) =
+          val evTyper = new Typer
+          val evCtx = ctx.fresh.setTyper(evTyper)
+          val evidence = evTyper.inferImplicitArg(tpt.tpe, tpt.span)(using evCtx)
+          evidence.tpe match
+            case fail: Implicits.SearchFailureType =>
+              val msg = evTyper.missingArgMsg(evidence, tpt.tpe, "")
+              errorTree(tpt, em"$msg")
+            case _ =>
+              evidence
+        return searchImplicit(callTypeArgs.head)
+      }
 
     def paramTypess(call: Tree, acc: List[List[Type]]): List[List[Type]] = call match
       case Apply(fn, args) =>

docs/docs/reference/metaprogramming/compiletime-ops.md

@@ -261,11 +261,30 @@ inline def f: Any = summonFrom {
 ## `summonInline`
 
 The shorthand `summonInline` provides a simple way to write a `summon` that is delayed until the call is inlined.
-
+Unlike `summonFrom`, `summonInline` also yields the implicit-not-found error, if a given instance of the summoned
+type is not found.
 ```scala
-transparent inline def summonInline[T]: T = summonFrom {
-  case t: T => t
-}
+import scala.compiletime.summonInline
+import scala.annotation.implicitNotFound
+
+@implicitNotFound("Missing One")
+trait Missing1
+
+@implicitNotFound("Missing Two")
+trait Missing2
+
+trait NotMissing
+given NotMissing = ???
+
+transparent inline def summonInlineCheck[T <: Int](inline t : T) : Any =
+  inline t match
+    case 1 => summonInline[Missing1]
+    case 2 => summonInline[Missing2]
+    case _ => summonInline[NotMissing]
+
+val missing1 = summonInlineCheck(1) // error: Missing One
+val missing2 = summonInlineCheck(2) // error: Missing Two
+val notMissing : NotMissing = summonInlineCheck(3)
 ```
 
 ## Reference

library/src/scala/compiletime/package.scala

@@ -137,9 +137,8 @@ transparent inline def summonFrom[T](f: Nothing => T): T =
  *  @tparam T the type of the value to be summoned
  *  @return the given value typed as the provided type parameter
  */
-transparent inline def summonInline[T]: T = summonFrom {
-  case t: T => t
-}
+transparent inline def summonInline[T]: T =
+  error("Compiler bug: `summonInline` was not evaluated by the compiler")
 
 /** Given a tuple T, summons each of its member types and returns them in
  *  a Tuple.

tests/neg/summonInline.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/summonInline.scala:19:32 ---------------------------------------------------------------------------
+19 |val missing1 = summonInlineCheck(1) // error
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |               Missing One
+   | This location contains code that was inlined from summonInline.scala:15
+-- Error: tests/neg/summonInline.scala:20:32 ---------------------------------------------------------------------------
+20 |val missing2 = summonInlineCheck(2) // error
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |               Missing Two
+   | This location contains code that was inlined from summonInline.scala:16

tests/neg/summonInline.scala

@@ -0,0 +1,21 @@
+import scala.compiletime.summonInline
+import scala.annotation.implicitNotFound
+
+@implicitNotFound("Missing One")
+trait Missing1
+
+@implicitNotFound("Missing Two")
+trait Missing2
+
+trait NotMissing
+given NotMissing = ???
+
+transparent inline def summonInlineCheck[T <: Int](inline t : T) : Any = 
+  inline t match 
+    case 1 => summonInline[Missing1]
+    case 2 => summonInline[Missing2]
+    case _ => summonInline[NotMissing]
+
+val missing1 = summonInlineCheck(1) // error
+val missing2 = summonInlineCheck(2) // error
+val notMissing : NotMissing = summonInlineCheck(3)