addded to type report output of tree, which can't be typed, because in macros pos can point to complete other thing.

Author: rssh

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

@@ -242,7 +242,7 @@ import transform.SymUtils._
     }
   }
 
-  class TypeMismatch(found: Type, expected: Type, addenda: => String*)(using Context)
+  class TypeMismatch(found: Type,  expected: Type, inTree: Option[untpd.Tree],  addenda: => String*)(using Context)
     extends TypeMismatchMsg(found, expected)(TypeMismatchID):
 
     // replace constrained TypeParamRefs and their typevars by their bounds where possible
@@ -281,7 +281,13 @@ import transform.SymUtils._
       val (foundStr, expectedStr) = Formatting.typeDiff(found2, expected2)(using printCtx)
       s"""|Found:    $foundStr
           |Required: $expectedStr""".stripMargin
-        + whereSuffix + postScript
+        + whereSuffix + postScript 
+
+    override def explain = 
+      val treeStr = inTree.map(x => s"\nTree: ${x.show}").getOrElse("")
+      treeStr + "\n" + err.whyNoMatchStr(found, expected)
+      
+
   end TypeMismatch
 
   class NotAMember(site: Type, val name: Name, selected: String, addendum: => String = "")(using Context)

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -592,7 +592,7 @@ class TreeChecker extends Phase with SymTransformer {
           !isPrimaryConstructorReturn &&
           !pt.isInstanceOf[FunOrPolyProto])
         assert(tree.tpe <:< pt, {
-          val mismatch = TypeMismatch(tree.tpe, pt)
+          val mismatch = TypeMismatch(tree.tpe, pt, Some(tree))
           i"""|${mismatch.msg}
               |found: ${infoStr(tree.tpe)}
               |expected: ${infoStr(pt)}

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

@@ -440,7 +440,7 @@ trait Applications extends Compatibility {
             // it might be healed by an implicit conversion
             ()
           else
-            fail(TypeMismatch(methType.resultType, resultType))
+            fail(TypeMismatch(methType.resultType, resultType, None))
 
         // match all arguments with corresponding formal parameters
         matchArgs(orderedArgs, methType.paramInfos, 0)

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

@@ -131,7 +131,7 @@ object ErrorReporting {
         case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
           "\nMaybe you are missing an else part for the conditional?"
         case _ => ""
-      errorTree(tree, TypeMismatch(treeTp, pt, implicitFailure.whyNoConversion, missingElse))
+      errorTree(tree, TypeMismatch(treeTp, pt, Some(tree), implicitFailure.whyNoConversion, missingElse))
     }
 
     /** A subtype log explaining why `found` does not conform to `expected` */

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

@@ -3880,7 +3880,7 @@ class Typer extends Namer
           // - it complicates the protocol
           // - such code patterns usually implies hidden errors in the code
           // - it's safe/sound to reject the code
-          report.error(TypeMismatch(tree.tpe, pt, "\npattern type is incompatible with expected type"), tree.srcPos)
+          report.error(TypeMismatch(tree.tpe, pt, Some(tree), "\npattern type is incompatible with expected type"), tree.srcPos)
         else
           val cmp =
             untpd.Apply(