Better error message for errors arising from implicit completions

Fixes scala#11994

Author: odersky

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

@@ -3066,30 +3066,30 @@ class Typer extends Namer
       for err <- nestedCtx.reporter.allErrors.take(1) do
         rememberSearchFailure(qual,
           SearchFailure(app.withType(FailedExtension(app, selectionProto, err.msg))))
+
+      // try an implicit conversion or given extension
+      if ctx.mode.is(Mode.ImplicitsEnabled) && !tree.name.isConstructorName && qual.tpe.isValueType then
+        trace(i"try insert impl on qualifier $tree $pt") {
+          val selProto = selectionProto
+          inferView(qual, selProto) match
+            case SearchSuccess(found, _, _, isExtension) =>
+              if isExtension then return found
+              else
+                checkImplicitConversionUseOK(found)
+                return typedSelect(tree, pt, found)
+            case failure: SearchFailure =>
+              if failure.isAmbiguous then
+                return (
+                  if canDefineFurther(qual.tpe.widen) then
+                    tryExtensionOrConversion(tree, pt, mbrProto, qual, locked, compat, privateOK)
+                  else
+                    err.typeMismatch(qual, selProto, failure.reason) // TODO: report NotAMember instead, but need to be aware of failure
+                )
+              rememberSearchFailure(qual, failure)
+        }
     catch case ex: TypeError =>
       rememberSearchFailure(qual,
         SearchFailure(qual.withType(NestedFailure(ex.toMessage, selectionProto))))
-
-    // try an implicit conversion or given extension
-    if ctx.mode.is(Mode.ImplicitsEnabled) && !tree.name.isConstructorName && qual.tpe.isValueType then
-      trace(i"try insert impl on qualifier $tree $pt") {
-        val selProto = selectionProto
-        inferView(qual, selProto) match
-          case SearchSuccess(found, _, _, isExtension) =>
-            if isExtension then return found
-            else
-              checkImplicitConversionUseOK(found)
-              return typedSelect(tree, pt, found)
-          case failure: SearchFailure =>
-            if failure.isAmbiguous then
-              return (
-                if canDefineFurther(qual.tpe.widen) then
-                  tryExtensionOrConversion(tree, pt, mbrProto, qual, locked, compat, privateOK)
-                else
-                  err.typeMismatch(qual, selProto, failure.reason) // TODO: report NotAMember instead, but need to be aware of failure
-              )
-            rememberSearchFailure(qual, failure)
-      }
     EmptyTree
   end tryExtensionOrConversion
 

tests/neg/i11994.check

@@ -0,0 +1,14 @@
+-- [E008] Not Found Error: tests/neg/i11994.scala:1:28 -----------------------------------------------------------------
+1 |implicit def foo[T <: Tuple.meow]: Unit = ??? // error
+  |                      ^^^^^^^^^^
+  |                      type meow is not a member of object Tuple.
+  |                      Extension methods were tried, but the search failed with:
+  |
+  |                          Cyclic reference involving method foo
+-- [E008] Not Found Error: tests/neg/i11994.scala:2:18 -----------------------------------------------------------------
+2 |given [T <: Tuple.meow]: Unit = ??? // error
+  |            ^^^^^^^^^^
+  |            type meow is not a member of object Tuple.
+  |            Extension methods were tried, but the search failed with:
+  |
+  |                Cyclic reference involving method given_Unit

tests/neg/i11994.scala

@@ -0,0 +1,2 @@
+implicit def foo[T <: Tuple.meow]: Unit = ??? // error
+given [T <: Tuple.meow]: Unit = ??? // error