Fix explanation for lower bounds failures

Fixes scala#15575

Author: odersky

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1446,7 +1446,9 @@ import transform.SymUtils._
   }
 
   class DoesNotConformToBound(tpe: Type, which: String, bound: Type)(using Context)
-    extends TypeMismatchMsg(tpe, bound)(DoesNotConformToBoundID) {
+    extends TypeMismatchMsg(
+      if which == "lower" then bound else tpe,
+      if which == "lower" then tpe else bound)(DoesNotConformToBoundID) {
     def msg = em"Type argument ${tpe} does not conform to $which bound $bound"
   }
 

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

@@ -159,6 +159,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/i12650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i9517.scala", defaultOptions.and("-Xprint-types")),
       compileFile("tests/neg-custom-args/i11637.scala", defaultOptions.and("-explain")),
+      compileFile("tests/neg-custom-args/i15575.scala", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/interop-polytypes.scala", allowDeepSubtypes.and("-Yexplicit-nulls")),
       compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")),
       compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings"),

tests/neg-custom-args/i15575.check

@@ -0,0 +1,40 @@
+-- [E057] Type Mismatch Error: tests/neg-custom-args/i15575.scala:3:27 -------------------------------------------------
+3 |  def bar[T]: Unit = foo[T & Any] // error
+  |                           ^
+  |                           Type argument T & Any does not conform to lower bound Any
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | I tried to show that
+  |   Any
+  | conforms to
+  |   T & Any
+  | but the comparison trace ended with `false`:
+  |
+  |   ==> Any  <:  T & Any
+  |     ==> Any  <:  T
+  |     <== Any  <:  T = false
+  |   <== Any  <:  T & Any = false
+  |
+  | The tests were made under the empty constraint
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E057] Type Mismatch Error: tests/neg-custom-args/i15575.scala:7:14 -------------------------------------------------
+7 |  val _ = foo[String]  // error
+  |              ^
+  |              Type argument String does not conform to lower bound CharSequence
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | I tried to show that
+  |   CharSequence
+  | conforms to
+  |   String
+  | but the comparison trace ended with `false`:
+  |
+  |   ==> CharSequence  <:  String
+  |     ==> CharSequence  <:  String
+  |     <== CharSequence  <:  String = false
+  |   <== CharSequence  <:  String = false
+  |
+  | The tests were made under the empty constraint
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg-custom-args/i15575.scala

@@ -0,0 +1,7 @@
+object Test1:
+  def foo[T >: Any]: Unit = ()
+  def bar[T]: Unit = foo[T & Any] // error
+
+object Test2:
+  def foo[T >: CharSequence]: Unit = ()
+  val _ = foo[String]  // error