Pruning branch of constant type condition.

As done in inliner.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -9,7 +9,7 @@ import dotty.tools.dotc.transform.TreeTransforms._
 import ast.Trees._
 import Flags._
 import Types._
-import Constants.Constant
+import Constants._
 import Contexts.Context
 import Symbols._
 import SymDenotations._
@@ -34,6 +34,8 @@ import StdNames._
  *   - drops branches of ifs using the rules
  *          if (true) A else B  --> A
  *          if (false) A else B --> B
+ *          if (C: true) A else B  --> C; A
+ *          if (C: false) A else B  --> C; B
  */
 class FirstTransform extends MiniPhaseTransform with InfoTransformer with AnnotationTransformer { thisTransformer =>
   import ast.tpd._
@@ -190,11 +192,16 @@ 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
+  override def transformIf(tree: If)(implicit ctx: Context, info: TransformerInfo) = {
+    tree.cond.tpe.widenTermRefExpr match {
+      case ConstantType(Constant(condVal: Boolean)) =>
+        val selected = if (condVal) tree.thenp else tree.elsep
+        if (isPureExpr(tree.cond)) selected
+        else Block(tree.cond :: Nil, selected)
+      case _ =>
+        tree
     }
+  }
 
   // invariants: all modules have companion objects
   // all types are TypeTrees

tests/run/if-with-constant-cond.check

@@ -0,0 +1,4 @@
+cond1
+then1
+cond2
+else2

tests/run/if-with-constant-cond.scala

@@ -0,0 +1,9 @@
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    if ({ println("cond1"); true }: true) println("then1") else println("else1")
+    if ({ println("cond2"); false }: false) println("then2") else println("else2")
+  }
+
+}