A warning for recursive calls that use defaults

Unfortunately it's a bit noisy, as this isn't always a bug.

Author: retronym

src/compiler/scala/tools/nsc/settings/Warnings.scala

@@ -55,6 +55,7 @@ trait Warnings {
   val warnNullaryOverride  = BooleanSetting   ("-Ywarn-nullary-override",
     "Warn when non-nullary overrides nullary, e.g. `def foo()` over `def foo`.")
   val warnInferAny         = BooleanSetting   ("-Ywarn-infer-any", "Warn when a type argument is inferred to be `Any`.")
+  val warnDefaultedRecurse = BooleanSetting   ("-Ywarn-default-recursion", "Warn when a recursive method call relies on a default argument")
 
   // Backward compatibility.
   def Xwarnfatal    = fatalWarnings

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -3071,7 +3071,13 @@ trait Typers extends Modes with Adaptations with Tags {
                   // useful when a default doesn't match parameter type, e.g. def f[T](x:T="a"); f[Int]()
                   val note = "Error occurred in an application involving default arguments."
                   if (!(context.diagnostic contains note)) context.diagnostic = note :: context.diagnostic
-                  doTypedApply(tree, if (blockIsEmpty) fun else fun1, allArgs, mode, pt)
+                  val resultTree = doTypedApply(tree, if (blockIsEmpty) fun else fun1, allArgs, mode, pt)
+
+                  if (settings.warnDefaultedRecurse.value && !resultTree.isErroneous && (context.owner.ownerChain contains fun.symbol))
+
+                    unit.warning(tree.pos, s"recursive call relies on defaults for some parameters")
+
+                  resultTree
                 } else {
                   tryTupleApply getOrElse duplErrorTree(NotEnoughArgsError(tree, fun, missing))
                 }

test/files/neg/recursive-method-default.check

@@ -0,0 +1,27 @@
+recursive-method-default.scala:4: warning: recursive call relies on defaults for some parameters
+    rec1(0)     // warn
+        ^
+recursive-method-default.scala:9: warning: recursive call relies on defaults for some parameters
+      rec2(0)   // warn
+          ^
+recursive-method-default.scala:12: warning: recursive call relies on defaults for some parameters
+      rec2(0)   // warn
+          ^
+recursive-method-default.scala:15: warning: recursive call relies on defaults for some parameters
+      rec2(0)   // warn
+          ^
+recursive-method-default.scala:26: warning: recursive call relies on defaults for some parameters
+    rec4(0)()   // warn
+           ^
+recursive-method-default.scala:30: warning: recursive call relies on defaults for some parameters
+    rec5(0)()   // warn
+           ^
+recursive-method-default.scala:34: warning: recursive call relies on defaults for some parameters
+    rec6()(0)   // warn
+        ^
+recursive-method-default.scala:38: warning: recursive call relies on defaults for some parameters
+    rec7()(0)   // one warning only
+        ^
+error: No warnings can be incurred under -Xfatal-warnings.
+8 warnings found
+one error found

test/files/neg/recursive-method-default.flags

@@ -0,0 +1 @@
+-Ywarn-default-recursion -Xfatal-warnings

test/files/neg/recursive-method-default.scala

@@ -0,0 +1,40 @@
+object Test {
+  def rec1(a: Any, b: Any = "".reverse): Any = {
+    rec1(0, 0)  // okay
+    rec1(0)     // warn
+  }
+
+  def rec2(a: Any, b: Any = "".reverse): Any = {
+    def nested {
+      rec2(0)   // warn
+    }
+    object X {
+      rec2(0)   // warn
+    }
+    class X {
+      rec2(0)   // warn
+    }
+  }
+
+
+  def rec3(a: Any) {}
+  def rec3(a: Any, b: Any = "".reverse): Any = {
+    rec3(0)     // okay
+  }
+
+  def rec4(a: Any)(b: Any = "".reverse): Any = {
+    rec4(0)()   // warn
+  }
+
+  def rec5(a: Any)(b: Any = 0): Any = {
+    rec5(0)()   // warn
+  }
+
+  def rec6(a: Any = 0)(b: Any = 0): Any = {
+    rec6()(0)   // warn
+  }
+
+  def rec7(a: Any = 0)(b: Any = 0): Any = {
+    rec7()(0)   // one warning only
+  }
+}