Fix #5386: Normalize unary operator expressions

A unary operator expression `pre.op` where `op` is one of `+`, `-`, `~`, `!`
that has a constant type `ConstantType(v)` but that is not a constant expression
(i.e. `pre` has side-effects) is translated to

    { pre; v }

This avoids the situation where we have a Select node which does not have a symbol.

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -449,7 +449,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   def isIdempotentRef(tree: Tree)(implicit ctx: Context): Boolean =
     refPurity(tree) >= Idempotent
 
-  /** If `tree` is a constant expression, its value as a Literal,
+  /** (1) If `tree` is a constant expression, its value as a Literal,
    *  or `tree` itself otherwise.
    *
    *  Note: Demanding idempotency instead of purity in literalize is strictly speaking too loose.
@@ -485,11 +485,27 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    *        Ident
    *        Select
    *        TypeApply
+   *
+   *  (2) A unary operator expression `pre.op` where `op` is one of `+`, `-`, `~`, `!`
+   *  that has a constant type `ConstantType(v)` but that is not a constant expression
+   *  (i.e. `pre` has side-effects) is translated to
+   *
+   *     { pre; v }
+   *
+   *  This avoids the situation where we have a Select node that does not have a symbol.
    */
   def constToLiteral(tree: Tree)(implicit ctx: Context): Tree = {
     val tree1 = ConstFold(tree)
     tree1.tpe.widenTermRefExpr match {
-      case ConstantType(value) if isIdempotentExpr(tree1) => Literal(value)
+      case ConstantType(value) =>
+        if (isIdempotentExpr(tree1)) Literal(value)
+        else tree1 match {
+          case Select(qual, _) if tree1.tpe.isInstanceOf[ConstantType] =>
+            // it's a unary operator; Simplify `pre.op` to `{ pre; v }` where `v` is the value of `pre.op`
+            Block(qual :: Nil, Literal(value))
+          case _ =>
+            tree1
+        }
       case _ => tree1
     }
   }

tests/pos/i5386.scala

@@ -0,0 +1,25 @@
+object Test {
+  ~{
+    println("!")
+    1
+  }
+
+  +{
+    println("!")
+    1
+  }
+
+  -{
+    println("!")
+    1
+  }
+
+  !{
+    println("!")
+    true
+  }
+
+  { println("1"); 1 } + { println("2"); 2}
+
+  !(try true finally{()})
+}