Propagate implicit search errors from implicit macros

Fixes partially #16835

Author: nicolasstucki

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

@@ -567,6 +567,12 @@ object Implicits:
 
     def msg(using Context) = em"Failed to synthesize an instance of type ${clarify(expectedType)}:${formatReasons}"
 
+  class ErrorReportedImplicit(errors: List[Diagnostic.Error],
+                           val expectedType: Type,
+                           val argument: Tree) extends SearchFailureType {
+    def msg(using Context): Message =
+      em"${errors.map(_.msg).mkString("\n")}"
+  }
 end Implicits
 
 import Implicits._
@@ -1157,19 +1163,22 @@ trait Implicits:
       if ctx.reporter.hasErrors
          || !cand.ref.symbol.isAccessibleFrom(cand.ref.prefix)
       then
-        ctx.reporter.removeBufferedMessages
-        adapted.tpe match {
+        val res = adapted.tpe match {
           case _: SearchFailureType => SearchFailure(adapted)
           case error: PreviousErrorType if !adapted.symbol.isAccessibleFrom(cand.ref.prefix) =>
             SearchFailure(adapted.withType(new NestedFailure(error.msg, pt)))
-          case _ =>
+          case tpe =>
             // Special case for `$conforms` and `<:<.refl`. Showing them to the users brings
             // no value, so we instead report a `NoMatchingImplicitsFailure`
             if (adapted.symbol == defn.Predef_conforms || adapted.symbol == defn.SubType_refl)
               NoMatchingImplicitsFailure
+            else if tpe <:< pt then
+              SearchFailure(adapted.withType(new ErrorReportedImplicit(ctx.reporter.allErrors.reverse, pt, argument)))
             else
               SearchFailure(adapted.withType(new MismatchedImplicit(ref, pt, argument)))
         }
+        ctx.reporter.removeBufferedMessages
+        res
       else
         SearchSuccess(adapted, ref, cand.level, cand.isExtension)(ctx.typerState, ctx.gadt)
     }

tests/neg-macros/i16835.check

@@ -0,0 +1,6 @@
+
+-- Error: tests/neg-macros/i16835/Test_2.scala:1:17 --------------------------------------------------------------------
+1 |def test: Unit = foo // error
+  |                 ^^^
+  |                 my error
+  |                 my second error

tests/neg-macros/i16835/Macro_1.scala

@@ -0,0 +1,21 @@
+import scala.quoted.*
+
+class Bar
+
+inline def foo: Unit = ${ fooExpr }
+
+def fooExpr(using Quotes): Expr[Unit] =
+  import quotes.reflect.*
+  Implicits.search(TypeRepr.of[Bar]) match
+    case res: ImplicitSearchSuccess   => '{}
+    case failure: ImplicitSearchFailure =>
+      report.errorAndAbort(failure.explanation)
+
+
+inline given bar: Bar = ${ barExpr }
+
+def barExpr(using Quotes): Expr[Bar] =
+  import quotes.reflect.*
+  report.error(s"my error")
+  report.error(s"my second error")
+  '{ new Bar }

tests/neg-macros/i16835/Test_2.scala

@@ -0,0 +1 @@
+def test: Unit = foo // error