Better error message for ifs that miss an else branch

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -241,7 +241,7 @@ object messages {
     }
   }
 
-  class TypeMismatch(found: Type, expected: Type, addendum: => String = "")(implicit ctx: Context)
+  class TypeMismatch(found: Type, expected: Type, addenda: => String*)(implicit ctx: Context)
     extends TypeMismatchMsg(TypeMismatchID):
 
     // replace constrained TypeParamRefs and their typevars by their bounds where possible
@@ -269,14 +269,15 @@ object messages {
       val expected1 = reported(expected)
       val (found2, expected2) =
         if (found1 frozen_<:< expected1) (found, expected) else (found1, expected1)
-      val postScript =
-        if !addendum.isEmpty
-           || expected.isAny
-           || expected.isAnyRef
-           || expected.isRef(defn.AnyValClass)
-           || defn.isBottomType(found)
-        then addendum
-        else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
+      val postScript = addenda.find(!_.isEmpty) match
+        case Some(p) => p
+        case None =>
+          if expected.isAny
+             || expected.isAnyRef
+             || expected.isRef(defn.AnyValClass)
+             || defn.isBottomType(found)
+          then ""
+          else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
       val (where, printCtx) = Formatting.disambiguateTypes(found2, expected2)
       val whereSuffix = if (where.isEmpty) where else s"\n\n$where"
       val (foundStr, expectedStr) = Formatting.typeDiff(found2, expected2)(printCtx)

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
@@ -98,7 +98,11 @@ object ErrorReporting {
       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))
+      def missingElse = tree match
+        case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
+          "\nMaybe you are missing an else part for the conditional?"
+        case _ => ""
+      errorTree(tree, TypeMismatch(treeTp, pt, implicitFailure.whyNoConversion, missingElse))
     }
 
     /** 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

tests/run/byname-varargs.check

@@ -0,0 +1,5 @@
+a
+b
+a
+b
+result = (ArraySeq(1, 2),ArraySeq(1, 2))

tests/run/byname-varargs.scala

@@ -0,0 +1,12 @@
+def f[T](xs: => T*): (Seq[T], Seq[T]) = (xs, xs)
+
+def a =
+  println("a")
+  1
+
+def b =
+  println("b")
+  2
+
+@main def Test =
+  println(s"result = ${f(a, b)}")