Warn on lossy conversion of literals & constants

Co-authored-by: Dale Wijnand <[email protected]>

Author: SethTisue

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -174,7 +174,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID]:
     OverrideTypeMismatchErrorID,
     OverrideErrorID,
     MatchableWarningID,
-    CannotExtendFunctionID
+    CannotExtendFunctionID,
+    LossyWideningConstantConversionID
 
   def errorNumber = ordinal - 2
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -807,6 +807,13 @@ import transform.SymUtils._
            |"""
   }
 
+  class LossyWideningConstantConversion(sourceType: Type, targetType: Type)(using Context)
+  extends Message(LossyWideningConstantConversionID):
+    def kind = "Lossy Conversion"
+    def msg = em"""|Widening conversion from $sourceType to $targetType loses precision.
+                   |Write `.to$targetType` instead.""".stripMargin
+    def explain = ""
+
   class PatternMatchExhaustivity(uncoveredFn: => String, hasMore: Boolean)(using Context)
   extends Message(PatternMatchExhaustivityID) {
     def kind = "Pattern Match Exhaustivity"

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

@@ -719,7 +719,11 @@ class Typer extends Namer
       else if (target.isRef(defn.FloatClass))
         tree.kind match {
           case Whole(16) => // cant parse hex literal as float
-          case _         => return lit(floatFromDigits(digits))
+          case _         =>
+            val float = floatFromDigits(digits)
+            if digits.toIntOption.exists(_ != float.toInt) then
+              report.warning(LossyWideningConstantConversion(defn.IntType, target), tree.srcPos)
+            return lit(float)
         }
       else if (target.isRef(defn.DoubleClass))
         tree.kind match {
@@ -3733,6 +3737,12 @@ class Typer extends Namer
         case ConstantType(x) =>
           val converted = x.convertTo(pt)
           if converted != null && (converted ne x) then
+            val cls = pt.classSymbol
+            if x.tag == IntTag && cls == defn.FloatClass && x.intValue.toFloat.toInt != x.intValue
+              || x.tag == LongTag && cls == defn.FloatClass  && x.longValue.toFloat.toLong != x.longValue
+              || x.tag == LongTag && cls == defn.DoubleClass && x.longValue.toDouble.toLong != x.longValue
+            then
+              report.warning(LossyWideningConstantConversion(x.tpe, pt), tree.srcPos)
             return adaptConstant(tree, ConstantType(converted))
         case _ =>
 

tests/neg-custom-args/fatal-warnings/i11333.check

@@ -0,0 +1,30 @@
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:2:19 -------------------------------
+2 |  val f1: Float  = 123456789            // error
+  |                   ^^^^^^^^^
+  |                   Widening conversion from Int to Float loses precision.
+  |                   Write `.toFloat` instead.
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:3:19 -------------------------------
+3 |  val d1: Double = 1234567890123456789L // error
+  |                   ^^^^^^^^^^^^^^^^^^^^
+  |                   Widening conversion from Long to Double loses precision.
+  |                   Write `.toDouble` instead.
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:4:19 -------------------------------
+4 |  val f2: Float  = 123456789L           // error
+  |                   ^^^^^^^^^^
+  |                   Widening conversion from Long to Float loses precision.
+  |                   Write `.toFloat` instead.
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:10:21 ------------------------------
+10 |  val f1_b: Float  = i1 // error
+   |                     ^^
+   |                     Widening conversion from Int to Float loses precision.
+   |                     Write `.toFloat` instead.
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:11:21 ------------------------------
+11 |  val d1_b: Double = l1 // error
+   |                     ^^
+   |                     Widening conversion from Long to Double loses precision.
+   |                     Write `.toDouble` instead.
+-- [E167] Lossy Conversion Error: tests/neg-custom-args/fatal-warnings/i11333.scala:12:21 ------------------------------
+12 |  val f2_b: Float  = l2 // error
+   |                     ^^
+   |                     Widening conversion from Long to Float loses precision.
+   |                     Write `.toFloat` instead.

tests/neg-custom-args/fatal-warnings/i11333.scala

@@ -0,0 +1,12 @@
+class C:
+  val f1: Float  = 123456789            // error
+  val d1: Double = 1234567890123456789L // error
+  val f2: Float  = 123456789L           // error
+
+  inline val i1 = 123456789
+  inline val l1 = 1234567890123456789L
+  inline val l2 = 123456789L
+
+  val f1_b: Float  = i1 // error
+  val d1_b: Double = l1 // error
+  val f2_b: Float  = l2 // error