Fix #2077: Optimization of constant conditionals

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

Author: odersky

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

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

@@ -739,11 +739,7 @@ import RefChecks._
  *
  *  2. It warns about references to symbols labeled deprecated or migration.
 
- *  3. It performs the following transformations:
- *
- *  - if (true) A else B  --> A
- *    if (false) A else B --> B
- *  - macro definitions are eliminated.
+ *  3. It eliminates macro definitions.
  *
  *  4. It makes members not private where necessary. The following members
  *  cannot be private in the Java model:
@@ -836,12 +832,6 @@ class RefChecks extends MiniPhase { thisTransformer =>
       tree
     }
 
-    override def transformIf(tree: If)(implicit ctx: Context, info: TransformerInfo) =
-      tree.cond.tpe match {
-        case ConstantType(value) => if (value.booleanValue) tree.thenp else tree.elsep
-        case _ => tree
-      }
-
     override def transformNew(tree: New)(implicit ctx: Context, info: TransformerInfo) = {
       currentLevel.enterReference(tree.tpe.typeSymbol, tree.pos)
       tree

tests/run/i2077.check

@@ -0,0 +1 @@
+hi

tests/run/i2077.scala

@@ -0,0 +1,10 @@
+object Test {
+  inline val x = true
+  val y = if (x) 1 else 2  // reduced to val y = 1
+
+  def main(args: Array[String]): Unit  =
+    if ({ println("hi"); true })  // cannot be reduced
+      1
+    else
+      2
+}