Take into account the result type of inline implicit conversions unle…

…ss they are transparent (#17924)

In the first commit, I add a failing test:

~~~ scala
import scala.language.implicitConversions

class Foo

object Foo:
  inline implicit def toFoo(x: Int): Foo = Foo()

object Usage:
  1.asdf // error
~~~

We expect that code to not compile but the error reported by the
compiler is confusing as it suggests importing `Foo.toFoo` to resolve
the compilation error. You can see this in the [test
report](https://github.com/lampepfl/dotty/actions/runs/5254687053/jobs/9493612604#step:9:1859).

The problem comes from the fact that currently when the compiler checks
whether an implicit conversion is applicable to an expression that fails
to compile, it does not take into account the expected result type
(here, `? { def asdf: ? }`) if the candidate is an `inline` definition.

Instead, I believe the expected result type should be taken into account
unless the candidate is a `transparent inline`. I make this change in
the second commit, which makes the test pass.

Fixes #9685

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

@@ -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
       }

tests/neg-macros/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-macros/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