From 2fc7aaea57bf7ac7cac8e92a50d3cfa71e5129dc Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Tue, 5 Dec 2023 09:33:25 +0100
Subject: [PATCH] Improve `asExprOf` cast error formatting

The intention of this change is to make it simpler to read the error
message.

List the expected type and actual type before the expression. This
is usually the most important information and simpler to parse as the
expression can get quite long. For the expected type, the actual type
and the expression, we print the value in the same line if its String
representation has only one line. Otherwise we print it in the next line
with an indentation and extra new line at the end.

Before:
```
java.lang.Exception: Expr cast exception: ((a: scala.Int) => ({
  val v: scala.Int = a
  Binding.apply[scala.Unit](())
}: Binding[scala.Unit]))
of type: scala.Function1[scala.Int, scala.Unit]
did not conform to type: scala.Function1[scala.Int, Binding[scala.Unit]]

    at scala.quoted.runtime.impl.QuotesImpl.asExprOf(QuotesImpl.scala:76)
    ...
```

After:
```
scala.quoted.runtime.impl.ExprCastException:
  Expected type: scala.Function1[scala.Int, scala.Unit]
  Actual type: scala.Function1[scala.Int, Binding[scala.Unit]]
  Expression:
    ((a: scala.Int) => ({
      val v: scala.Int = a
      Binding.apply[scala.Unit](())
    }: Binding[scala.Unit]))

    at scala.quoted.runtime.impl.QuotesImpl.asExprOf(QuotesImpl.scala:...)
    ...
```
---
 .../runtime/impl/ExprCastException.scala      | 20 +++++++++++++++++++
 .../quoted/runtime/impl/QuotesImpl.scala      | 10 +++++-----
 2 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 compiler/src/scala/quoted/runtime/impl/ExprCastException.scala

diff --git a/compiler/src/scala/quoted/runtime/impl/ExprCastException.scala b/compiler/src/scala/quoted/runtime/impl/ExprCastException.scala
new file mode 100644
index 000000000000..ba870808cee3
--- /dev/null
+++ b/compiler/src/scala/quoted/runtime/impl/ExprCastException.scala
@@ -0,0 +1,20 @@
+package scala.quoted.runtime.impl
+
+import dotty.tools.dotc.ast.tpd.Tree
+import dotty.tools.dotc.core.Contexts.*
+
+class ExprCastException(msg: String) extends Exception(msg)
+
+
+object ExprCastException:
+  def apply(expectedType: String, actualType: String, exprCode: String): ExprCastException =
+    new ExprCastException(
+      s"""|
+          |  Expected type: ${formatLines(expectedType)}
+          |  Actual type: ${formatLines(actualType)}
+          |  Expression: ${formatLines(exprCode)}
+          |""".stripMargin)
+
+  private def formatLines(str: String): String =
+    if !str.contains("\n") then str
+    else str.linesIterator.mkString("\n    ", "\n    ", "\n")
diff --git a/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala b/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
index 04472647b9fc..6aa279129f2b 100644
--- a/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
+++ b/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala
@@ -23,6 +23,7 @@ import scala.quoted.runtime.{QuoteUnpickler, QuoteMatching}
 import scala.quoted.runtime.impl.printers.*
 
 import scala.reflect.TypeTest
+import dotty.tools.dotc.core.NameKinds.ExceptionBinderName
 
 object QuotesImpl {
 
@@ -70,11 +71,10 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
       if self.isExprOf[X] then
         self.asInstanceOf[scala.quoted.Expr[X]]
       else
-        throw Exception(
-          s"""Expr cast exception: ${self.show}
-            |of type: ${reflect.Printer.TypeReprCode.show(reflect.asTerm(self).tpe)}
-            |did not conform to type: ${reflect.Printer.TypeReprCode.show(reflect.TypeRepr.of[X])}
-            |""".stripMargin
+        throw ExprCastException(
+          expectedType = reflect.Printer.TypeReprCode.show(reflect.TypeRepr.of[X]),
+          actualType = reflect.Printer.TypeReprCode.show(reflect.asTerm(self).tpe),
+          exprCode = self.show
         )
     }
   end extension