Better error message: reassignment to val

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

@@ -1551,17 +1551,25 @@ class AmbiguousExtensionMethod(tree: untpd.Tree, expansion1: tpd.Tree, expansion
        |are possible expansions of $tree"""
   def explain(using Context) = ""
 
-class ReassignmentToVal(name: Name)(using Context)
+class ReassignmentToVal(name: Name, pt: Type)(using Context)
   extends TypeMsg(ReassignmentToValID) {
-  def msg(using Context) = i"""Reassignment to val $name"""
-  def explain(using Context) =
-    i"""|You can not assign a new value to $name as values can't be changed.
-        |Keep in mind that every statement has a value, so you may e.g. use
-        |  ${hl("val")} $name ${hl("= if (condition) 2 else 5")}
-        |In case you need a reassignable name, you can declare it as
-        |variable
-        |  ${hl("var")} $name ${hl("=")} ...
-        |"""
+
+  def booleanExpected = pt.isRef(defn.BooleanClass)
+  def booleanNote =
+    if booleanExpected then
+      ". Maybe you meant to write an equality test using `==`?"
+    else ""
+  def msg(using Context)
+    = i"""Reassignment to val $name$booleanNote"""
+
+  def explain(using Context) =
+    if booleanExpected then
+      i"""An equality test is written "x == y" using `==`. A single `=` stands
+         |for assigment, where a variable on the left gets a new value on the right.
+         |This is only permitted if the variable is declared with `var`."""
+    else
+      i"""You can not assign a new value to $name as values can't be changed.
+         |Reassigment is only permitted if the variable is declared with `var`."""
 }
 
 class TypeDoesNotTakeParameters(tpe: Type, params: List[untpd.Tree])(using Context)

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

@@ -137,7 +137,7 @@ trait Dynamic {
       case TypeApply(sel @ Select(qual, name), targs) if !isDynamicMethod(name) =>
         typedDynamicAssign(qual, name, sel.span, targs)
       case _ =>
-        errorTree(tree, ReassignmentToVal(tree.lhs.symbol.name))
+        errorTree(tree, ReassignmentToVal(tree.lhs.symbol.name, pt))
     }
   }
 

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

@@ -1429,7 +1429,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         def lhs1 = adapt(lhsCore, LhsProto, locked)
 
         def reassignmentToVal =
-          report.error(ReassignmentToVal(lhsCore.symbol.name), tree.srcPos)
+          report.error(ReassignmentToVal(lhsCore.symbol.name, pt), tree.srcPos)
           cpy.Assign(tree)(lhsCore, typed(tree.rhs, lhs1.tpe.widen)).withType(defn.UnitType)
 
         def canAssign(sym: Symbol) =

tests/neg/reassignment.check

@@ -0,0 +1,53 @@
+-- [E052] Type Error: tests/neg/reassignment.scala:4:10 ----------------------------------------------------------------
+4 |val y = x = 0  // error
+  |        ^^^^^
+  |        Reassignment to val x
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | You can not assign a new value to x as values can't be changed.
+  | Reassigment is only permitted if the variable is declared with `var`.
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E052] Type Error: tests/neg/reassignment.scala:5:19 ----------------------------------------------------------------
+5 |val z: Boolean = x = 0 // error
+  |                 ^^^^^
+  |                 Reassignment to val x. Maybe you meant to write an equality test using `==`?
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | An equality test is written "x == y" using `==`. A single `=` stands
+  | for assigment, where a variable on the left gets a new value on the right.
+  | This is only permitted if the variable is declared with `var`.
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E052] Type Error: tests/neg/reassignment.scala:6:13 ----------------------------------------------------------------
+6 |def f = if x = 0 then 1 else 2  // error
+  |           ^^^^^
+  |           Reassignment to val x. Maybe you meant to write an equality test using `==`?
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | An equality test is written "x == y" using `==`. A single `=` stands
+  | for assigment, where a variable on the left gets a new value on the right.
+  | This is only permitted if the variable is declared with `var`.
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E052] Type Error: tests/neg/reassignment.scala:8:4 -----------------------------------------------------------------
+8 |  x = 0  // error
+  |  ^^^^^
+  |  Reassignment to val x
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | You can not assign a new value to x as values can't be changed.
+  | Reassigment is only permitted if the variable is declared with `var`.
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E052] Type Error: tests/neg/reassignment.scala:9:4 -----------------------------------------------------------------
+9 |  x = 2  // error
+  |  ^^^^^
+  |  Reassignment to val x. Maybe you meant to write an equality test using `==`?
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | An equality test is written "x == y" using `==`. A single `=` stands
+  | for assigment, where a variable on the left gets a new value on the right.
+  | This is only permitted if the variable is declared with `var`.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/reassignment.scala

@@ -0,0 +1,10 @@
+//> using options -explain
+
+val x = 1
+val y = x = 0  // error
+val z: Boolean = x = 0 // error
+def f = if x = 0 then 1 else 2  // error
+def g: Boolean =
+  x = 0  // error
+  x = 2  // error
+