Do not suggest implicits from scalajs.js.|

Now that we unpickle Scala.js unions as actual unions, these implicits
are suddenly applicable in many more situations and can end up polluting
error messages with implicit suggestions since they should never be used
explicitly, so blacklist them from the suggestions.

Author: smarter

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

@@ -2,11 +2,13 @@ package dotty.tools
 package dotc
 package typer
 
+import backend.sjs.JSDefinitions
 import core._
 import Contexts._, Types._, Symbols._, Names._, Decorators._, ProtoTypes._
 import Flags._, SymDenotations._
 import NameKinds.FlatName
 import NameOps._
+import StdNames._
 import config.Printers.{implicits, implicitsDetailed}
 import util.Spans.Span
 import ast.{untpd, tpd}
@@ -64,6 +66,8 @@ trait ImportSuggestions:
         else !root.name.is(FlatName)
           && !root.name.lastPart.contains('$')
           && root.is(ModuleVal, butNot = JavaDefined)
+          // The implicits in `scalajs.js.|` are implementation details and shouldn't be suggested
+          && !(root.name == nme.raw.BAR && ctx.settings.scalajs.value && root == JSDefinitions.jsdefn.PseudoUnionModule)
       }
 
     def nestedRoots(site: Type)(using Context): List[Symbol] =

tests/neg-scalajs/type-mismatch.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg-scalajs/type-mismatch.scala:6:17 ----------------------------------------------
+6 |    val n: Int = msg // error message shouldn't mention implicits from `scalajs.js.|`
+  |                 ^^^
+  |                 Found:    (msg : String)
+  |                 Required: Int
+
+longer explanation available when compiling with `-explain`

tests/neg-scalajs/type-mismatch.scala

@@ -0,0 +1,8 @@
+import scala.scalajs.js._
+
+object HelloWorld {
+  def main(args: Array[String]): Unit = {
+    val msg = "hello"
+    val n: Int = msg // error message shouldn't mention implicits from `scalajs.js.|`
+  }
+}