Describe and add tests for source incompabilities

Author: odersky

docs/_docs/reference/experimental/named-tuples.md

@@ -174,3 +174,24 @@ This revives SIP 43, with a much simpler desugaring than originally proposed.
 Named patterns are compatible with extensible pattern matching simply because
 `unapply` results can be named tuples.
 
+### Source Incompatibilities
+
+There are some source incompatibilities involving named tuples of length one.
+First, what was previously classified as an assignment could now be interpreted as a named tuple. Example:
+```scala
+var age: Int
+(age = 1)
+```
+This was an assignment in parentheses before, and is a named tuple of arity one now. It is however not idiomatic Scala code, since assignments are not usually enclosed in parentheses.
+
+Second, what was a named argument to an infix operator can now be interpreted as a named tuple.
+```scala
+class C:
+  infix def f(age: Int)
+val c: C
+```
+then
+```scala
+c f (age = 1)
+```
+will now construct a tuple as second operand instead of passing a named parameter.

tests/pos/namedtuple-src-incompat.scala

@@ -0,0 +1,17 @@
+import language.experimental.namedTuples
+var age = 22
+val x = (age = 1)
+val _: (age: Int) = x
+val x2 = {age = 1}
+val _: Unit = x2
+
+class C:
+  infix def id[T](age: T): T = age
+
+def test =
+  val c: C = ???
+  val y = c id (age = 1)
+  val _: (age: Int) = y
+  val y2 = c.id(age = 1)
+  val _: Int = y2
+