Add new rule to outdentation based on scala/scala3#10331

Author: tgodzik

scalameta/parsers/shared/src/main/scala/scala/meta/internal/parsers/ScalametaParser.scala

@@ -348,7 +348,7 @@ class ScalametaParser(input: Input, dialect: Dialect) { parser =>
             while ((indentedRegion(sepRegionsProcess)
                 && sepRegionsProcess.head.indent > currentIndent && !isLeadingInfixOperator(
                   curr
-                )) ||
+                ) && !prev.is[CanContinueOnNextLine]) ||
               shouldCloseCaseOnNonCase(sepRegionsProcess)) {
               insertOutdent()
               // match can start an identation, block but if `match` follows it means it's chaining matches
@@ -880,6 +880,15 @@ class ScalametaParser(input: Input, dialect: Dialect) { parser =>
       (token.next.strictNext.is[LineEnd] || token.next.strictNext.is[EOF])
     }
   }
+  // then  else  do  catch  finally  yield  match
+  @classifier
+  trait CanContinueOnNextLine {
+    def unapply(token: Token): Boolean = {
+      token.is[KwThen] || token.is[KwElse] || token.is[KwDo] || 
+      token.is[KwCatch] || token.is[KwFinally] || token.is[KwYield] ||
+      token.is[KwMatch]
+    }
+  }
 
   @classifier
   trait ExprIntro {

tests/shared/src/test/scala/scala/meta/tests/parsers/dotty/SignificantIndentationSuite.scala

@@ -101,7 +101,7 @@ class SignificantIndentationSuite extends BaseDottySuite {
     )
   }
 
-  test("indent-below-not-okay".ignore) {
+  test("then-no-indent") {
     // this test is related to dotty issue: https://github.com/lampepfl/dotty/issues/9790
     // It should either assert error during parsing: "illegal start of simple expression"
     // Or accept mismatch with parsing rules and parse as 'if (cond) { truep } else {falsep }'
@@ -112,17 +112,41 @@ class SignificantIndentationSuite extends BaseDottySuite {
                   |    else
                   |  falsep
                   |""".stripMargin
-    runTestAssert[Stat](code, assertLayout = Some("trait A { def f: Int }"))(
-      Defn.Trait(
+    runTestAssert[Stat](
+      code,
+      assertLayout = Some(
+        """|def fn: Unit = {
+           |  if (cond) truep else falsep
+           |}
+           |""".stripMargin
+      )
+    )(
+      Defn.Def(
         Nil,
-        Type.Name("A"),
+        Term.Name("fn"),
         Nil,
-        Ctor.Primary(Nil, Name(""), Nil),
-        Template(Nil, Nil, Self(Name(""), None), List(defx))
+        Nil,
+        Some(Type.Name("Unit")),
+        Term.Block(List(Term.If(Term.Name("cond"), Term.Name("truep"), Term.Name("falsep"))))
       )
     )
   }
 
+  test("then-no-indent-wrong") {
+    // this test is related to dotty issue: https://github.com/lampepfl/dotty/issues/9790
+    // It should either assert error during parsing: "illegal start of simple expression"
+    // Or accept mismatch with parsing rules and parse as 'if (cond) { truep } else {falsep }'
+    // Why error is thrown is described in mentioned issue.
+    val code = """|def fn: Unit =
+                  |    if cond then
+                  |  truep1
+                  |  truep2
+                  |    else
+                  |  falsep
+                  |""".stripMargin
+    runTestError[Stat](code, "expected but else found")
+  }
+
   test("indent-inside-brace-ok") {
     val code = """|class X {
                   |  def fx(): Unit =