Warn on misleading indentation in single-case catch

Fixes #14721

Surprisingly, caught two instances in the compiler code base itself.

Author: odersky

compiler/src/dotty/tools/backend/jvm/BytecodeWriters.scala

@@ -118,7 +118,7 @@ trait BytecodeWriters {
         try
           outfile.delete() // don't leave an empty or half-written classfile around after an interrupt
         catch case _: Throwable =>
-        throw ex
+          throw ex
       finally outstream.close()
       report.informProgress("wrote '" + label + "' to " + outfile)
     }

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -276,7 +276,7 @@ class GenBCodePipeline(val int: DottyBackendInterface, val primitives: DottyPrim
               try
                 outTastyFile.delete() // don't leave an empty or half-written tastyfile around after an interrupt
               catch case _: Throwable =>
-              throw ex
+                throw ex
             finally outstream.close()
 
             val uuid = new TastyHeaderUnpickler(binary()).readHeader()

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

@@ -2620,7 +2620,12 @@ object Parsers {
         (pattern(), guard())
       }
       CaseDef(pat, grd, atSpan(accept(ARROW)) {
-        if exprOnly then expr() else block()
+        if exprOnly then
+          if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
+            warning(i"""misleading indentation: this expression forms part of the preceding catch case,
+                       |it should be indented for clarity.""")
+          expr()
+        else block()
       })
     }
 

tests/neg-custom-args/fatal-warnings/i14721.scala

@@ -0,0 +1,9 @@
+import scala.util.chaining.given
+
+class C:
+  def op: Unit = println("op")
+  def handler: Unit = println("handler")
+  def test: Unit =
+    try op
+    catch case _: NullPointerException =>
+    handler   // error