Adjust the fix for scala#9685 to increase source compatibility.

We apply the full fix under `-source future` only, and a partial fix otherwise.

This is a followup of scala#17924 that fixes the source incompatibilities reported in scala#18123.

We test that the behavior under `-source future` still fixes scala#9685, and that without `-source future` we fix both scala#9685 and scala#18123.

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

@@ -18,6 +18,7 @@ import ErrorReporting.*
 import util.SourceFile
 import TypeComparer.necessarySubType
 import dotty.tools.dotc.core.Flags.Transparent
+import dotty.tools.dotc.config.{ Feature, SourceVersion }
 
 import scala.annotation.internal.sharable
 
@@ -113,9 +114,22 @@ object ProtoTypes {
      *  achieved by replacing expected type parameters with wildcards.
      */
     def constrainResult(meth: Symbol, mt: Type, pt: Type)(using Context): Boolean =
-      if (Inlines.isInlineable(meth) && meth.is(Transparent)) {
-        constrainResult(mt, wildApprox(pt))
-        true
+      if (Inlines.isInlineable(meth)) {
+        // Stricter behaviour in 3.4+: do not apply `wildApprox` to non-transparent inlines
+        if (Feature.sourceVersion.isAtLeast(SourceVersion.future)) {
+          if (meth.is(Transparent)) {
+            constrainResult(mt, wildApprox(pt))
+            // do not constrain the result type of transparent inline methods
+            true
+          } else {
+            constrainResult(mt, pt)
+          }
+        } else {
+          // Best-effort to fix https://github.com/lampepfl/dotty/issues/9685 in the 3.3.x series
+          // while preserving source compatibility as much as possible
+          val methodMatchedType = constrainResult(mt, wildApprox(pt))
+          meth.is(Transparent) || methodMatchedType
+        }
       }
       else constrainResult(mt, pt)
   }

compiler/test/dotty/tools/dotc/BootstrappedOnlyCompilationTests.scala

@@ -109,6 +109,7 @@ class BootstrappedOnlyCompilationTests {
     implicit val testGroup: TestGroup = TestGroup("compileNegWithCompiler")
     aggregateTests(
       compileFilesInDir("tests/neg-macros", defaultOptions.and("-Xcheck-macros")),
+      compileFilesInDir("tests/neg-inlines-strict", defaultOptions.and("-source", "future")),
       compileFile("tests/pos-macros/i9570.scala", defaultOptions.and("-Xfatal-warnings")),
       compileFile("tests/pos-macros/macro-deprecation.scala", defaultOptions.and("-Xfatal-warnings", "-deprecation")),
       compileFile("tests/pos-macros/macro-experimental.scala", defaultOptions.and("-Yno-experimental")),

tests/neg-inlines-strict/i9685bis.check

@@ -0,0 +1,9 @@
+-- [E008] Not Found Error: tests/neg-macros/i9685bis.scala:23:4 --------------------------------------------------------
+23 |  1.asdf // error
+   |  ^^^^^^
+   |  value asdf is not a member of Int, but could be made available as an extension method.
+   |
+   |  The following import might make progress towards fixing the problem:
+   |
+   |    import foo.Baz.toBaz
+   |

tests/neg-inlines-strict/i9685bis.scala

@@ -0,0 +1,23 @@
+package foo
+
+import scala.language.implicitConversions
+
+class Foo
+
+object Foo:
+
+  inline implicit def toFoo(x: Int): Foo = Foo()
+
+class Bar
+
+object Bar:
+  inline given Conversion[Int, Bar] with
+    def apply(x: Int): Bar = Bar()
+
+class Baz
+
+object Baz:
+  transparent inline implicit def toBaz(x: Int): Baz = Baz()
+
+object Usage:
+  1.asdf // error

tests/pos-macros/i18123.scala

@@ -0,0 +1,25 @@
+// may not compile anymore in Scala 3.4+
+package pkg
+
+trait P[+T]
+
+extension [T](inline parse0: P[T])
+  inline def | [V >: T](inline other: P[V]): P[V] = ???
+
+extension [T](inline parse0: => P[T])
+  inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
+
+object Implicits:
+  trait Repeater[-T, R]
+  object Repeater:
+    implicit def GenericRepeaterImplicit[T]: Repeater[T, Seq[T]] = ???
+
+sealed trait RegexTree
+abstract class Node extends RegexTree
+class CharClassIntersection() extends Node
+
+def classItem: P[RegexTree] = ???
+def charClassIntersection: P[CharClassIntersection] = ???
+
+def x =
+  (charClassIntersection.rep() | classItem.rep())