Take into account the result type of inline implicit conversions unless they are transparent

Fixes #9685

Author: julienrf

community-build/community-projects/munit

@@ -17,6 +17,7 @@ import Inferencing.*
 import ErrorReporting.*
 import util.SourceFile
 import TypeComparer.necessarySubType
+import dotty.tools.dotc.core.Flags.Transparent
 
 import scala.annotation.internal.sharable
 
@@ -105,14 +106,14 @@ object ProtoTypes {
       if !res then ctx.typerState.constraint = savedConstraint
       res
 
-    /** Constrain result with special case if `meth` is an inlineable method in an inlineable context.
+    /** Constrain result with special case if `meth` is a transparent inlineable method in an inlineable context.
      *  In that case, we should always succeed and not constrain type parameters in the expected type,
      *  because the actual return type can be a subtype of the currently known return type.
      *  However, we should constrain parameters of the declared return type. This distinction is
      *  achieved by replacing expected type parameters with wildcards.
      */
     def constrainResult(meth: Symbol, mt: Type, pt: Type)(using Context): Boolean =
-      if (Inlines.isInlineable(meth)) {
+      if (Inlines.isInlineable(meth) && meth.is(Transparent)) {
         constrainResult(mt, wildApprox(pt))
         true
       }

community-build/community-projects/specs2

@@ -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
+   |

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