Constant fold String.==

compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala

@@ -11,6 +11,7 @@ import dotty.tools.dotc.core.NameKinds._
 import dotty.tools.dotc.core.Symbols._
 import dotty.tools.dotc.core.Types._
 import dotty.tools.dotc.transform.MegaPhase.MiniPhase
+import dotty.tools.dotc.typer.ConstFold
 
 /**
   * MiniPhase to transform s and raw string interpolators from using StringContext to string
@@ -162,6 +163,18 @@ class StringInterpolatorOpt extends MiniPhase {
           }
       }
     else
-      tree
+      tree.tpe match
+        case _: ConstantType => tree
+        case _ =>
+          ConstFold.Apply(tree).tpe match
+            case ConstantType(x) => Literal(x).withSpan(tree.span).ensureConforms(tree.tpe)
+            case _ => tree
   }
+
+  override def transformSelect(tree: Select)(using Context): Tree = {
+    ConstFold.Select(tree).tpe match
+      case ConstantType(x) => Literal(x).withSpan(tree.span).ensureConforms(tree.tpe)
+      case _ => tree
+  }
+
 }

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

tests/run-macros/tasty-extractors-1.check

@@ -73,8 +73,8 @@ ConstantType(IntConstant(2))
 Inlined(None, Nil, Try(Literal(IntConstant(3)), List(CaseDef(Ident("_"), None, Block(Nil, Literal(UnitConstant())))), Some(Literal(UnitConstant()))))
 OrType(ConstantType(IntConstant(3)), TypeRef(ThisType(TypeRef(NoPrefix(), "scala")), "Unit"))
 
-Inlined(None, Nil, Apply(Select(Literal(StringConstant("a")), "=="), List(Literal(StringConstant("b")))))
-TypeRef(ThisType(TypeRef(NoPrefix(), "scala")), "Boolean")
+Inlined(None, Nil, Literal(BooleanConstant(false)))
+ConstantType(BooleanConstant(false))
 
 Inlined(None, Nil, Apply(Select(New(TypeIdent("Object")), "<init>"), Nil))
 TypeRef(ThisType(TypeRef(NoPrefix(), "lang")), "Object")