Fix #7742 : Ensure the indentWidth is never null

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

@@ -1359,7 +1359,10 @@ object Scanners {
 
   case class InBraces(var width: IndentWidth | Null, outer: Region)
   extends IndentSignificantRegion {
-    override def indentWidth = width
+    // The indent width starts out as `null` when the opening brace is encountered
+    // It is then adjusted when the next token on a new line is encountered.
+    override def indentWidth: IndentWidth =
+      if width == null then IndentWidth.Zero else width
   }
 
   /** A class describing an indentation region.

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -77,8 +77,11 @@ class TreeChecker extends Phase with SymTransformer {
       testDuplicate(sym, seenClasses, "class")
     }
 
-    val isDeferredAndPrivate = sym.is(Method) && sym.is(Deferred) && sym.is(Private)
-    assert(!isDeferredAndPrivate, i"$sym is both Deferred and Private")
+    val badDeferredAndPrivate =
+      sym.is(Method) && sym.is(Deferred) && sym.is(Private)
+      && !sym.hasAnnotation(defn.NativeAnnot)
+      && !sym.is(Erased)
+    assert(!badDeferredAndPrivate, i"$sym is both Deferred and Private")
 
     checkCompanion(symd)
 

tests/neg/i7742.scala

@@ -0,0 +1,3 @@
+object A {
+    for // error // error
+}

tests/neg/i7753.scala

@@ -0,0 +1,2 @@
+class A { if 0 // error
+} // error

tests/pos/i7741.scala

@@ -0,0 +1,9 @@
+class A1 {
+    @native private def a: Unit
+}
+trait A2 {
+    erased def i(a: Int): Int
+}
+trait A3 {
+    erased val a: Int
+}