Fix #2077: Optimization of constant conditionals

Move fixed logic to FirstTransform, where the other constant
folding operations are also done.

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala b/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
index 597146514..9e71fda5d 100644
--- a/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
+++ b/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
@@ -31,6 +31,9 @@ import StdNames._
  *   - eliminates self tree in Template and self symbol in ClassInfo
  *   - collapsess all type trees to trees of class TypeTree
  *   - converts idempotent expressions with constant types
+ *   - drops branches of ifs using the rules
+ *          if (true) A else B  --> A
+ *          if (false) A else B --> B
  */
 class FirstTransform extends MiniPhaseTransform with InfoTransformer with AnnotationTransformer { thisTransformer =>
   import ast.tpd._
@@ -187,6 +190,12 @@ class FirstTransform extends MiniPhaseTransform with InfoTransformer with Annota
   override def transformBlock(tree: Block)(implicit ctx: Context, info: TransformerInfo) =
     constToLiteral(tree)
 
+  override def transformIf(tree: If)(implicit ctx: Context, info: TransformerInfo) =
+    tree.cond match {
+      case Literal(Constant(c: Boolean)) => if (c) tree.thenp else tree.elsep
+      case _ => tree
+    }
+
   // invariants: all modules have companion objects
   // all types are TypeTrees
   // all this types are explicit