Only postprocess by-name args

The previous code was buggy in that *all* arguments of a method
containing a single by-name argument were post processed.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Nullables.scala

@@ -507,7 +507,10 @@ object Nullables with
 
             def recur(formals: List[Type], args: List[Tree]): List[Tree] = (formals, args) match
               case (formal :: formalsRest, arg :: argsRest) =>
-                val arg1 = postProcess(formal.widenExpr.repeatedToSingle, arg)
+                val arg1 =
+                  if formal.isInstanceOf[ExprType]
+                  then postProcess(formal.widenExpr.repeatedToSingle, arg)
+                  else arg
                 val argsRest1 = recur(
                   if formal.isRepeatedParam then formals else formalsRest,
                   argsRest)

tests/neg-custom-args/explicit-nulls/byname-nullables1.check

@@ -1,8 +1,8 @@
--- Error: tests/neg-custom-args/explicit-nulls/byname-nullables1.scala:9:22 --------------------------------------------
-9 |  if x != null then f(x.fld != null) // error
-  |                      ^^^^^^^^^^^^^
-  |                      This argument was typed using flow assumptions about mutable variables
-  |                      but it is passed to a by-name parameter where such flow assumptions are unsound.
-  |                      Wrapping the argument in `byName(...)` fixes the problem by disabling the flow assumptions.
-  |
-  |                      `byName` needs to be imported from the `scala.compiletime` package.
+-- Error: tests/neg-custom-args/explicit-nulls/byname-nullables1.scala:10:6 --------------------------------------------
+10 |    f(x.fld != null) // error
+   |      ^^^^^^^^^^^^^
+   |      This argument was typed using flow assumptions about mutable variables
+   |      but it is passed to a by-name parameter where such flow assumptions are unsound.
+   |      Wrapping the argument in `byName(...)` fixes the problem by disabling the flow assumptions.
+   |
+   |      `byName` needs to be imported from the `scala.compiletime` package.

tests/neg-custom-args/explicit-nulls/byname-nullables1.scala

@@ -6,4 +6,6 @@ class C with
 
 def test() =
   var x: C | Null = C()
-  if x != null then f(x.fld != null) // error
+  if x != null then
+    f(x.fld != null) // error
+    require(x.fld != null, "error")    // ok