Refine condition when to report errors in an inserted apply

Fixes #15000

Author: odersky

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

@@ -3184,12 +3184,26 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ => false
     }
 
+    var assumeApplyExists = false
+      // if true, issue any errors about the apply instead of `fallBack`,
+      // since they are more likely to be informative.
+
     def tryApply(using Context) = {
       val pt1 = pt.withContext(ctx)
       val sel = typedSelect(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt1)
         .withAttachment(InsertedApply, ())
-      if (sel.tpe.isError) sel
-      else try adapt(simplify(sel, pt1, locked), pt1, locked) finally sel.removeAttachment(InsertedApply)
+      if sel.tpe.isError then
+        // assume the apply exists if qualifier has a hidden search failure of type
+        // FailedExtension or NestedFailure
+        sel match
+          case Select(qual, _) =>
+            assumeApplyExists = qual.getAttachment(Typer.HiddenSearchFailure).exists(
+              _.exists(_.reason.isInstanceOf[FailedExtension | NestedFailure]))
+          case _ =>
+        sel
+      else
+        assumeApplyExists = true
+        try adapt(simplify(sel, pt1, locked), pt1, locked) finally sel.removeAttachment(InsertedApply)
     }
 
     def tryImplicit(fallBack: => Tree) =
@@ -3209,11 +3223,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         if (isApplyProto(pt) || isMethod(tree) || isSyntheticApply(tree)) tryImplicit(fallBack)
         else tryEither(tryApply) { (app, appState) =>
           tryImplicit {
-            if (tree.tpe.member(nme.apply).exists) {
-              // issue the error about the apply, since it is likely more informative than the fallback
+            if assumeApplyExists then
               appState.commit()
               app
-            }
             else fallBack
           }
         }

tests/neg/i15000.check

@@ -0,0 +1,24 @@
+-- [E006] Not Found Error: tests/neg/i15000.scala:19:11 ----------------------------------------------------------------
+19 |    str(x, barY) // error
+   |           ^^^^
+   |           Not found: barY
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/i15000.scala:20:9 -----------------------------------------------------------------
+20 |    c(x, barY) // error // error
+   |         ^^^^
+   |         Not found: barY
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E008] Not Found Error: tests/neg/i15000.scala:20:4 -----------------------------------------------------------------
+20 |    c(x, barY) // error // error
+   |    ^
+   |value apply is not a member of object ExtensionMethodReproduction.c.
+   |An extension method was tried, but could not be fully constructed:
+   |
+   |    apply(ExtensionMethodReproduction.c)    failed with
+   |
+   |        Ambiguous overload. The overloaded alternatives of method apply in object ExtensionMethodReproduction with types
+   |         (c: ExtensionMethodReproduction.C)(x: Int, y: Int): String
+   |         (c: ExtensionMethodReproduction.C)(x: Int, y: String): String
+   |        both match arguments (ExtensionMethodReproduction.c.type)

tests/neg/i15000.scala

@@ -0,0 +1,21 @@
+object ExtensionMethodReproduction {
+    import scala.language.implicitConversions
+    class StringExtensions {
+      def apply(x: Int, y: String): String = "foo"
+      def apply(x: Int, y: Int): String = "foo"
+    }
+    implicit def stringExtensions(m: String): StringExtensions = new StringExtensions()
+
+    class C
+    object c extends C
+
+    extension (c: C)
+      def apply(x: Int, y: String): String = "foo"
+      def apply(x: Int, y: Int): String = "foo"
+
+    val str = "POST"
+    val x: Int = 1
+    val fooY: String = "foo"
+    str(x, barY) // error
+    c(x, barY) // error // error
+  }