Added warning when someone tries to return a no...

Added warning when someone tries to return a non-Unit value from a Unit
method, no review.

4 files changed, 29 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 8718d31520..add3bca6f2 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3141,14 +3141,20 @@ trait Typers extends Modes {
             ) {
           errorTree(tree, "return outside method definition")
         } else {
-          val DefDef(_, _, _, _, restpt, _) = enclMethod.tree
-          var restpt0 = restpt
-          if (restpt0.tpe eq null) {
-            errorTree(tree, "" + enclMethod.owner +
-                      " has return statement; needs result type")
-          } else {
+          val DefDef(_, name, _, _, restpt, _) = enclMethod.tree
+          if (restpt.tpe eq null)
+            errorTree(tree, enclMethod.owner + " has return statement; needs result type")
+          else {
             context.enclMethod.returnsSeen = true
-            val expr1: Tree = typed(expr, EXPRmode | BYVALmode, restpt0.tpe)
+            val expr1: Tree = typed(expr, EXPRmode | BYVALmode, restpt.tpe)
+            // Warn about returning a value if no value can be returned.
+            if (restpt.tpe.typeSymbol == UnitClass) {
+              // The typing in expr1 says expr is Unit (it has already been coerced if
+              // it is non-Unit) so we have to retype it.  Fortunately it won't come up much
+              // unless the warning is legitimate.
+              if (typed(expr).tpe.typeSymbol != UnitClass)
+                unit.warning(tree.pos, "enclosing method " + name + " has result type Unit: return value discarded")
+            }
             treeCopy.Return(tree, checkDead(expr1)) setSymbol enclMethod.owner setType NothingClass.tpe
           }
         }
diff --git a/test/files/neg/unit-returns-value.check b/test/files/neg/unit-returns-value.check
new file mode 100644
index 0000000000..18368f45ab
--- /dev/null
+++ b/test/files/neg/unit-returns-value.check
@@ -0,0 +1,4 @@
+unit-returns-value.scala:4: error: enclosing method f has result type Unit: return value discarded
+    if (b) return 5
+           ^
+one error found
diff --git a/test/files/neg/unit-returns-value.flags b/test/files/neg/unit-returns-value.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/unit-returns-value.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/unit-returns-value.scala b/test/files/neg/unit-returns-value.scala
new file mode 100644
index 0000000000..27a1111c5b
--- /dev/null
+++ b/test/files/neg/unit-returns-value.scala
@@ -0,0 +1,11 @@
+object Test {
+  def f {
+    var b = false
+    if (b) return 5
+  }
+
+  // no warning
+  def g {
+    return println("hello")
+  }
+}