SI-1980 A lint warning for by-name parameters in right assoc methods

The desugaring of right associative calls happens in the parser. This
eagerly evaluates the arguments (to preserve left-to-right evaluation
order the arguments are evaluated before the qualifier).

This is pretty surprising if the method being called has a by-name
parameter in the first parameter section.

This commit adds a warning under -Xlint when defining such a method.

The relevent spec snippets:

> SLS 4.6.1 says that call-by-name argument "is not evaluated at the point of function application, but instead is evaluated at each use within the function".
>
> But 6.12.3 offers:
> "If op is right- associative, the same operation is interpreted as { val x=e1; e2.op(x ) }, where x is a fresh name."

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/neg/t1980.scala b/test/files/neg/t1980.scala
new file mode 100644
index 0000000000..132865e694
--- /dev/null
+++ b/test/files/neg/t1980.scala
@@ -0,0 +1,9 @@
+object Test {
+  def op1_:(x: => Any) = ()                 // warn
+  def op2_:(x: Any, y: => Any) = ()         // warn
+  def op3_:(x: Any, y: => Any)(a: Any) = () // warn
+
+  def op4() = ()                            // no warn
+  def op5(x: => Any) = ()                   // no warn
+  def op6_:(x: Any)(a: => Any) = ()         // no warn
+}