Boolean format is null test for non-Boolean

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala

@@ -219,7 +219,7 @@ class TypedFormatChecker(partsElems: List[Tree], parts: List[String], args: List
     def acceptableVariants: List[Type] =
       kind match
         case StringXn        => if hasFlag('#') then FormattableType :: Nil else defn.AnyType :: Nil
-        case BooleanXn       => defn.BooleanType :: defn.NullType :: Nil
+        case BooleanXn       => defn.BooleanType :: defn.NullType :: defn.AnyType :: Nil // warn if not boolean
         case HashXn          => defn.AnyType :: Nil
         case CharacterXn     => defn.CharType :: defn.ByteType :: defn.ShortType :: defn.IntType :: Nil
         case IntegralXn      => defn.IntType :: defn.LongType :: defn.ByteType :: defn.ShortType :: BigIntType :: Nil

tests/neg/f-interpolator-neg.check

@@ -14,10 +14,10 @@
 7 |    new StringContext("", "").f() // error
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |    too few arguments for interpolated string
--- Error: tests/neg/f-interpolator-neg.scala:11:7 ----------------------------------------------------------------------
-11 |    f"$s%b" // error
-   |       ^
-   |       Found: (s : String), Required: Boolean, Null
+-- Warning: tests/neg/f-interpolator-neg.scala:11:9 --------------------------------------------------------------------
+11 |    f"$s%b" // warn only
+   |         ^
+   |         Boolean format is null test for non-Boolean
 -- Error: tests/neg/f-interpolator-neg.scala:12:7 ----------------------------------------------------------------------
 12 |    f"$s%c" // error
    |       ^

tests/neg/f-interpolator-neg.scala

@@ -8,7 +8,7 @@ object Test {
   }
 
   def interpolationMismatches(s : String, f : Double, b : Boolean) = {
-    f"$s%b" // error
+    f"$s%b" // warn only
     f"$s%c" // error
     f"$f%c" // error
     f"$s%x" // error

tests/warn/tostring-interpolated.scala

@@ -0,0 +1,36 @@
+//> using options -Wtostring-interpolated
+//> abusing options -Wconf:cat=w-flag-tostring-interpolated:e -Wtostring-interpolated
+
+case class C(x: Int)
+
+trait T {
+  def c = C(42)
+  def f = f"$c" // warn
+  def s = s"$c" // warn
+  def r = raw"$c" // warn
+
+  def format = f"${c.x}%d in $c or $c%s" // warn using c.toString // warn
+
+  def bool = f"$c%b" // warn just a null check
+
+  def oops = s"${null} slipped thru my fingers" // warn
+
+  def ok = s"${c.toString}"
+
+  def sb = new StringBuilder().append("hello")
+  def greeting = s"$sb, world" // warn
+}
+
+class Mitigations {
+
+  val s = "hello, world"
+  val i = 42
+  def shown = println("shown")
+
+  def ok = s"$s is ok"
+  def jersey = s"number $i"
+  def unitized = s"unfortunately $shown" // maybe tell them about unintended ()?
+
+  def nopct = f"$s is ok"
+  def nofmt = f"number $i"
+}