From 474552988e43ec1143493cc925106568b7404b0f Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Fri, 13 Jun 2025 11:36:32 -0700
Subject: [PATCH] Check OrType in interpolated toString lint

---
 .../transform/localopt/FormatChecker.scala    | 18 ++++++-
 .../localopt/StringInterpolatorOpt.scala      | 22 ++++++--
 tests/warn/tostring-interpolated.scala        | 53 +++++++++++++++++--
 3 files changed, 82 insertions(+), 11 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala b/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
index 42e5f0acb3f6..b628108d3a1d 100644
--- a/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/localopt/FormatChecker.scala
@@ -228,8 +228,22 @@ class TypedFormatChecker(partsElems: List[Tree], parts: List[String], args: List
       case _ => true
 
     def lintToString(arg: Type): Unit =
-      if ctx.settings.Whas.toStringInterpolated && kind == StringXn && !(arg.widen =:= defn.StringType) && !arg.isPrimitiveValueType
-      then warningAt(CC)("interpolation uses toString")
+      def check(tp: Type): Boolean = tp.widen match
+        case OrType(tp1, tp2) =>
+          check(tp1) || check(tp2)
+        case tp =>
+          if tp =:= defn.StringType then
+            false
+          else if tp =:= defn.UnitType then
+            warningAt(CC)("interpolated Unit value")
+            true
+          else if !tp.isPrimitiveValueType then
+            warningAt(CC)("interpolation uses toString")
+            true
+          else
+            false
+      if ctx.settings.Whas.toStringInterpolated && kind == StringXn && check(arg) then
+        ()
 
     // what arg type if any does the conversion accept
     def acceptableVariants: List[Type] =
diff --git a/compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala b/compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala
index 1afcfbac6206..37ae391325db 100644
--- a/compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala
+++ b/compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala
@@ -105,13 +105,27 @@ class StringInterpolatorOpt extends MiniPhase:
         lintToString(elem)
         concat(elem)
         val str = stri.next()
-        if !str.const.stringValue.isEmpty then concat(str)
+        if !str.const.stringValue.isEmpty then
+          concat(str)
       result
     end mkConcat
     def lintToString(t: Tree): Unit =
-      val arg: Type = t.tpe
-      if ctx.settings.Whas.toStringInterpolated && !(arg.widen =:= defn.StringType) && !arg.isPrimitiveValueType
-      then report.warning("interpolation uses toString", t.srcPos)
+      def check(tp: Type): Boolean = tp.widen match
+        case OrType(tp1, tp2) =>
+          check(tp1) || check(tp2)
+        case tp =>
+          if tp =:= defn.StringType then
+            false
+          else if tp =:= defn.UnitType then
+            report.warning("interpolated Unit value", t.srcPos)
+            true
+          else if !tp.isPrimitiveValueType then
+            report.warning("interpolation uses toString", t.srcPos)
+            true
+          else
+            false
+      if ctx.settings.Whas.toStringInterpolated && check(t.tpe) then
+        ()
     val sym = tree.symbol
     // Test names first to avoid loading scala.StringContext if not used, and common names first
     val isInterpolatedMethod =
diff --git a/tests/warn/tostring-interpolated.scala b/tests/warn/tostring-interpolated.scala
index 165bc374b5ef..f81dab3a518b 100644
--- a/tests/warn/tostring-interpolated.scala
+++ b/tests/warn/tostring-interpolated.scala
@@ -11,14 +11,18 @@ trait T {
 
   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
+
+  def literally = s"Hello, ${"world"}" // nowarn literal, widened to String
+
+  def bool = f"$c%b" // warn just a null check (quirk of Java format)
+
+  def oops = s"${null} slipped thru my fingers" // warn although conforms to String
+
+  def exceptionally = s"Hello, ${???}" // warn although conforms to String
 }
 
 class Mitigations {
@@ -29,8 +33,47 @@ class Mitigations {
 
   def ok = s"$s is ok"
   def jersey = s"number $i"
-  def unitized = s"unfortunately $shown" // maybe tell them about unintended ()?
+  def unitized = s"unfortunately $shown" // warn accidental unit value
+  def funitized = f"unfortunately $shown" // warn accidental unit value
 
   def nopct = f"$s is ok"
   def nofmt = f"number $i"
 }
+
+class Branches {
+
+  class C {
+    val shouldCaps = true
+    val greeting = s"Hello ${if (shouldCaps) "WORLD" else "world"}"
+  }
+
+  class D {
+    val shouldCaps = true
+    object world { override def toString = "world" }
+    val greeting = s"Hello ${if (shouldCaps) "WORLD" else world}" // warn
+  }
+
+  class E {
+    def x = 42
+    val greeting = s"Hello ${x match { case 42 => "WORLD" case 27 => "world" case _ => ??? }}"
+  }
+
+  class F {
+    def x = 42
+    object world { override def toString = "world" }
+    val greeting = s"Hello ${
+      x match { // warn
+        case 17 => "Welt"
+        case 42 => "WORLD"
+        case 27 => world
+        case _ => ??? }
+    }"
+  }
+
+  class Z {
+    val shouldCaps = true
+    val greeting = s"Hello ${if (shouldCaps) ??? else null}" // warn
+    val farewell = s"Bye-bye ${if (shouldCaps) "Bob" else null}" // warn
+  }
+
+}