Better error message for ifs that miss an else branch

Author: odersky

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

@@ -5,7 +5,7 @@ package typer
 import ast._
 import core._
 import Types._, ProtoTypes._, Contexts._, Decorators._, Denotations._, Symbols._
-import Implicits._, Flags._
+import Implicits._, Flags._, Constants.Constant
 import util.Spans._
 import util.SourcePosition
 import java.util.regex.Matcher.quoteReplacement
@@ -97,8 +97,14 @@ object ErrorReporting {
     def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailureType = NoMatchingImplicits): Tree = {
       val normTp = normalize(tree.tpe, pt)
       val treeTp = if (normTp <:< pt) tree.tpe else normTp
-        // use normalized type if that also shows an error, original type otherwise
-      errorTree(tree, TypeMismatch(treeTp, pt, implicitFailure.whyNoConversion))
+      // use normalized type if that also shows an error, original type otherwise
+      def missingElse = tree match
+        case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
+          "\nMaybe you are missing an else part for the conditional?"
+        case _ => ""
+      val addendum = List(implicitFailure.whyNoConversion, missingElse)
+        .find(!_.isEmpty).getOrElse("")
+      errorTree(tree, TypeMismatch(treeTp, pt, addendum))
     }
 
     /** A subtype log explaining why `found` does not conform to `expected` */

tests/neg/if-error.check

@@ -0,0 +1,6 @@
+-- [E007] Type Mismatch Error: tests/neg/if-error.scala:2:2 ------------------------------------------------------------
+2 |  if ??? then System.in.read()  // error
+  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |  Found:    Unit
+  |  Required: Int
+  |  Maybe you are missing an else part for the conditional?

tests/neg/if-error.scala

@@ -0,0 +1,2 @@
+val x: Int =
+  if ??? then System.in.read()  // error