Constant fold String.==

Author: nicolasstucki

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

@@ -163,6 +163,11 @@ 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.ADD  => Constant(x.stringValue + y.stringValue)
+    case nme.EQ  => Constant(x.stringValue == y.stringValue)
+    case _ => null
+  }
 
   private def foldBinop(op: Name, x: Constant, y: Constant): Constant =
     val optag =
@@ -176,7 +181,7 @@ 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 case ex: ArithmeticException => null // the code will crash at runtime,
                                                  // but that is better than the

tests/pos/i10521.scala

@@ -0,0 +1,8 @@
+transparent inline def default(inline name: String): Any =
+  inline if name == "Int" then 0
+  else inline if name == "String" then ""
+  else ???
+
+
+def test =
+  default("Int")

tests/pos/stringConstantFold.scala

@@ -0,0 +1,5 @@
+def test =
+  ("A" + "B": "AB")
+
+  ("A" == "A": true)
+  ("A" == "B": false)

tests/pos/t5856b.scala

@@ -0,0 +1,3 @@
+class Test:
+  def test = f("a" == s"a")
+  inline def f(inline b: Boolean): Boolean = !b