Rewrite underscore with optional space

Author: som-snytt

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

@@ -2007,15 +2007,19 @@ 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   ]) = ()