fix rewrites for filtering for generators

the test added here used to fail because the `case` was patched
inside the parens, e.g. `for (case x: String) ...` which is not valid

Author: griggt

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

@@ -1603,7 +1603,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 val isPatDef = checkMode == desugar.MatchCheck.IrrefutablePatDef
                 if (!checkIrrefutable(sel, pat, isPatDef) && sourceVersion == `future-migration`)
                   if (isPatDef) patch(Span(tree.selector.span.end), ": @unchecked")
-                  else patch(Span(pat.span.start), "case ")
+                  else patch(Span(tree.span.start), "case ")
 
                 // skip exhaustivity check in later phase
                 // TODO: move the check above to patternMatcher phase

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

@@ -73,6 +73,7 @@ class CompilationTests {
     aggregateTests(
       compileFile("tests/rewrites/rewrites.scala", scala2CompatMode.and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
+      compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/i8982.scala", defaultOptions.and("-indent", "-rewrite")),
       compileFile("tests/rewrites/i9632.scala", defaultOptions.and("-indent", "-rewrite")),
       compileFile("tests/rewrites/i11895.scala", defaultOptions.and("-indent", "-rewrite")),

tests/rewrites/filtering-fors.check

@@ -0,0 +1,6 @@
+val xs: List[Any] = ???
+val as = for case (x: String) <- xs yield x
+val bs =
+  for
+    case (x: String) <- xs
+  yield x

tests/rewrites/filtering-fors.scala

@@ -0,0 +1,6 @@
+val xs: List[Any] = ???
+val as = for (x: String) <- xs yield x
+val bs =
+  for
+    (x: String) <- xs
+  yield x