Make sure that fewerBraces as default will not work for 3.0-migration

Under 3.0-migration

    { x: T =>
      expr
    }

Is legal code but it's forbidden for later versions. If `fewerBraces` was the default, that code
would be ambiguous.

Author: odersky

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

@@ -214,7 +214,12 @@ object Scanners {
       if fewerBracesEnabledCtx ne myLanguageImportContext then
         fewerBracesEnabledCache =
           featureEnabled(Feature.fewerBraces)
-          || fewerBracesByDefault && indentSyntax
+          || fewerBracesByDefault && indentSyntax && !migrateTo3
+          		// ensure that fewer braces is not the default for 3.0-migration since
+          		//     { x: T =>
+          		//       expr
+          		//     }
+                // would be ambiguous
         fewerBracesEnabledCtx = myLanguageImportContext
       fewerBracesEnabledCache
 

tests/rewrites/rewrites.scala

@@ -56,7 +56,7 @@ object test3 {
       }
     }
   }
-  def g = { (x: Int) =>
+  def g = { x: Int =>
     x + 1
   }
 }