better feedback for SI-5044

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

@@ -1034,6 +1034,12 @@ trait ContextErrors {
         setError(arg)
       } else arg
     }
+
+    def WarnAfterNonSilentRecursiveInference(param: Symbol, arg: Tree)(implicit context: Context) = {
+      val note = "type-checking the invocation of "+ param.owner +" checks if the named argument expression '"+ param.name + " = ...' is a valid assignment\n"+
+                 "in the current scope. The resulting type inference error (see above) can be fixed by providing an explicit type in the local definition for "+ param.name +"."
+      context.warning(arg.pos, note)
+    }
 
     def UnknownParameterNameNamesDefaultError(arg: Tree, name: Name)(implicit context: Context) = {
       issueNormalTypeError(arg, "unknown parameter name: " + name)

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

@@ -478,7 +478,14 @@ trait NamesDefaults { self: Analyzer =>
         // instead of arg, but can't do that because eventually setType(ErrorType)
         // is called, and EmptyTree can only be typed NoType.  Thus we need to
         // disable conforms as a view...
-        try typer.silent(_.typed(arg, subst(paramtpe))) match {
+        val errsBefore = reporter.ERROR.count
+        try typer.silent { tpr =>
+          val res = tpr.typed(arg, subst(paramtpe))
+          // better warning for SI-5044: if `silent` was not actually silent give a hint to the user
+          if (errsBefore < reporter.ERROR.count)
+            WarnAfterNonSilentRecursiveInference(param, arg)(context)
+          res
+        } match {
           case SilentResultValue(t)  => !t.isErroneous // #4041
           case _        => false
         }
@@ -487,7 +494,7 @@ trait NamesDefaults { self: Analyzer =>
           // CyclicReferences.  Fix for #3685
           case cr @ CyclicReference(sym, _) =>
             (sym.name == param.name) && sym.accessedOrSelf.isVariable && {
-              NameClashError(sym, arg)(typer.context)
+              NameClashError(sym, arg)(context)
               true
             }
         }

test/files/neg/names-defaults-neg.check

@@ -149,8 +149,12 @@ names-defaults-neg.scala:170: error: reference to x is ambiguous; it is both a m
 names-defaults-neg.scala:177: error: variable definition needs type because 'x' is used as a named argument in its body.
   class u15 { var x = u.f(x = 1) }
                   ^
+names-defaults-neg.scala:177: warning: type-checking the invocation of method f checks if the named argument expression 'x = ...' is a valid assignment
+in the current scope. The resulting type inference error (see above) can be fixed by providing an explicit type in the local definition for x.
+  class u15 { var x = u.f(x = 1) }
+                            ^
 names-defaults-neg.scala:180: error: reference to x is ambiguous; it is both a method parameter and a variable in scope.
   class u18 { var x: Int = u.f(x = 1) }
                                  ^
-one warning found
+two warnings found
 41 errors found

test/files/neg/t5044.check

@@ -0,0 +1,9 @@
+t5044.scala:7: error: recursive value a needs type
+    val id = m(a)
+               ^
+t5044.scala:6: warning: type-checking the invocation of method foo checks if the named argument expression 'id = ...' is a valid assignment
+in the current scope. The resulting type inference error (see above) can be fixed by providing an explicit type in the local definition for id.
+    val a = foo(id = 1)
+                   ^
+one warning found
+one error found

test/files/neg/t5044.scala

@@ -0,0 +1,9 @@
+class T {
+  def foo[T](id: T) = 0
+  def m(a: Int) = 0
+
+  def f {
+    val a = foo(id = 1)
+    val id = m(a)
+  }
+}