Constant fold string equality

Author: OlivierBlanvillain

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

@@ -183,6 +183,12 @@ object ConstFold {
     case nme.MOD => Constant(x.doubleValue % y.doubleValue)
     case _ => null
   }
+  private def foldStringOp(op: Name, x: Constant, y: Constant): Constant = op match {
+    case nme.EQ  => Constant(x.stringValue == y.stringValue)
+    case nme.NE  => Constant(x.stringValue != y.stringValue)
+    case nme.ADD => Constant(x.stringValue + y.stringValue)
+    case _ => null
+  }
 
   private def foldBinop(op: Name, x: Constant, y: Constant): Constant = {
     val optag =
@@ -196,7 +202,7 @@ object ConstFold {
       case LongTag                                  => foldLongOp(op, x, y)
       case FloatTag                                 => foldFloatOp(op, x, y)
       case DoubleTag                                => foldDoubleOp(op, x, y)
-      case StringTag if op == nme.ADD               => Constant(x.stringValue + y.stringValue)
+      case StringTag                                => foldStringOp(op, x, y)
       case _                                        => null
     }
     catch {

tests/pos/dependent-eq.scala

@@ -0,0 +1,13 @@
+object Test {
+  def assertT(t: { true })  = ()
+  def assertF(f: { false }) = ()
+
+  dependent def r1 = 1 == 1    ; assertT(r1)
+  dependent def r2 = "1" =="1" ; assertT(r2)
+  dependent def r3 = 1 == 2    ; assertF(r3)
+  dependent def r4 = "1" =="2" ; assertF(r4)
+  dependent def r6 = 1 != 1    ; assertF(r6)
+  dependent def r7 = "1" !="1" ; assertF(r7)
+  dependent def r8 = 1 != 2    ; assertT(r8)
+  dependent def r9 = "1" !="2" ; assertT(r9)
+}