Don't allow implicit conversions on prefixes of type selections

Fixes #19888

Author: odersky

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

@@ -714,9 +714,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       val elems = qual.tpe.widenTermRefExpr.tupleElementTypes.getOrElse(Nil)
       typedSelect(tree, pt, qual.cast(defn.tupleType(elems)))
     else
-      val tree1 = tryExtensionOrConversion(
+      val tree1 = {
+        if selName.isTypeName then EmptyTree
+        else tryExtensionOrConversion(
           tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
-        .orElse {
+        }.orElse {
           if ctx.gadt.isNarrowing then
             // try GADT approximation if we're trying to select a member
             // Member lookup cannot take GADTs into account b/c of cache, so we

tests/neg/i11994.check

@@ -1,26 +1,12 @@
--- [E008] Not Found Error: tests/neg/i11994.scala:2:28 -----------------------------------------------------------------
-2 |implicit def foo[T <: Tuple.meow]: Unit = ??? // error
-  |                      ^^^^^^^^^^
-  |                      type meow is not a member of object Tuple.
-  |                      Extension methods were tried, but the search failed with:
-  |
-  |                          Cyclic reference involving method foo
-  |                          
-  |                          The error occurred while trying to compute the signature of method foo
-  |                            which required to compute the signature of type T
-  |                            which required to compute the signature of method foo
-  |                          
-  |                           Run with both -explain-cyclic and -Ydebug-cyclic to see full stack trace.
--- [E008] Not Found Error: tests/neg/i11994.scala:3:18 -----------------------------------------------------------------
+-- [E046] Cyclic Error: tests/neg/i11994.scala:3:18 --------------------------------------------------------------------
 3 |given [T <: Tuple.meow]: Unit = ??? // error
-  |            ^^^^^^^^^^
-  |            type meow is not a member of object Tuple.
-  |            Extension methods were tried, but the search failed with:
+  |                  ^
+  |                  Cyclic reference involving method given_Unit
+  |
+  |                  The error occurred while trying to compute the signature of given instance given_Unit
+  |                    which required to compute the signature of type T
+  |                    which required to compute the signature of given instance given_Unit
+  |
+  |                   Run with both -explain-cyclic and -Ydebug-cyclic to see full stack trace.
   |
-  |                Cyclic reference involving method given_Unit
-  |                
-  |                The error occurred while trying to compute the signature of given instance given_Unit
-  |                  which required to compute the signature of type T
-  |                  which required to compute the signature of given instance given_Unit
-  |                
-  |                 Run with both -explain-cyclic and -Ydebug-cyclic to see full stack trace.
+  | longer explanation available when compiling with `-explain`

tests/neg/i19888.scala

@@ -0,0 +1,16 @@
+object Main{
+  object Foo
+  object Bar{ type Qux = Unit }
+
+
+  implicit def conv(f: Foo.type): Bar.type = Bar
+
+  def B: Bar.type = Bar
+
+  def main(args: Array[String]): Unit = {
+    val x: Foo.Qux = null // error
+    val y: B.Qux = null
+    println(x)
+    println(y)
+  }
+}