Rewrite underscore with optional space (#23525)

Fixes #23449 

For the anomalous case where the usual space is missing, print the type
bounds for the patch.

This is not quite pretty printing, see the test of weird spaces inside
the brackets.

Author: hamzaremmal

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2006,15 +2006,19 @@ object Parsers {
           Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
         else
           if !inMatchPattern then
-            report.errorOrMigrationWarning(
-              em"`_` is deprecated for wildcard arguments of types: use `?` instead${rewriteNotice(`3.4-migration`)}",
-              in.sourcePos(),
-              MigrationVersion.WildcardType)
-            if MigrationVersion.WildcardType.needsPatch then
-              patch(source, Span(in.offset, in.offset + 1), "?")
-          end if
+            val msg =
+              em"`_` is deprecated for wildcard arguments of types: use `?` instead${rewriteNotice(`3.4-migration`)}"
+            report.errorOrMigrationWarning(msg, in.sourcePos(), MigrationVersion.WildcardType)
           val start = in.skipToken()
           typeBounds().withSpan(Span(start, in.lastOffset, start))
+            .tap: tbt =>
+              if !inMatchPattern && MigrationVersion.WildcardType.needsPatch then
+                val offset_? = tbt.span.start
+                if Chars.isOperatorPart(source(offset_? + 1)) then
+                  patch(source, tbt.span, "?" + ctx.printer.toText(tbt).mkString())
+                else
+                  patch(source, Span(offset_?, offset_? + 1), "?")
+
       // Allow symbols -_ and +_ through for compatibility with code written using kind-projector in Scala 3 underscore mode.
       // While these signify variant type parameters in Scala 2 + kind-projector, we ignore their variance markers since variance is inferred.
       else if (isIdent(nme.MINUS) || isIdent(nme.PLUS)) && in.lookahead.token == USCORE && ctx.settings.XkindProjector.value == "underscores" then

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

@@ -88,6 +88,7 @@ class CompilationTests {
       compileFile("tests/rewrites/i22731b.scala", defaultOptions.and("-rewrite", "-source:3.7-migration")),
       compileFile("tests/rewrites/implicit-to-given.scala", defaultOptions.and("-rewrite", "-Yimplicit-to-given")),
       compileFile("tests/rewrites/i22792.scala", defaultOptions.and("-rewrite")),
+      compileFile("tests/rewrites/i23449.scala", defaultOptions.and("-rewrite", "-source:3.4-migration")),
     ).checkRewrites()
   }
 

tests/rewrites/i23449.check

@@ -0,0 +1,14 @@
+trait T
+class C[A]
+
+def f(x: C[? <: T]) = ()
+
+def g(x: C[? >: T]) = ()
+
+def h(x: C[? <: T]) = ()
+
+def k(x: C[? >: T]) = ()
+
+def m(x: C[? >: Nothing <: T]) = ()
+
+def n(x: C[    ? >: Nothing <: T   ]) = ()

tests/rewrites/i23449.scala

@@ -0,0 +1,14 @@
+trait T
+class C[A]
+
+def f(x: C[_<:T]) = ()
+
+def g(x: C[_>:T]) = ()
+
+def h(x: C[_<: T]) = ()
+
+def k(x: C[_ >: T]) = ()
+
+def m(x: C[_>:Nothing<:T]) = ()
+
+def n(x: C[    _>:Nothing     <:T   ]) = ()