From a227ce2a197e4ca0884f517a62683960d3fcb88b Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Tue, 26 Dec 2023 16:24:32 -0800
Subject: [PATCH] Don't explain erroneous bounds

---
 .../dotty/tools/dotc/reporting/Message.scala   | 18 ++++++++----------
 tests/neg/i19334.check                         | 12 ++++++++++++
 tests/neg/i19334.scala                         |  6 ++++++
 3 files changed, 26 insertions(+), 10 deletions(-)
 create mode 100644 tests/neg/i19334.check
 create mode 100644 tests/neg/i19334.scala

diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index f9abead161d2..1ac5c6ecf407 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -76,8 +76,7 @@ object Message:
      *  and following recordings get consecutive superscripts starting with 2.
      *  @return  The possibly superscripted version of `str`.
      */
-    def record(str: String, isType: Boolean, entry: Recorded)(using Context): String =
-      if !recordOK then return str
+    def record(str: String, isType: Boolean, entry: Recorded)(using Context): String = if !recordOK then str else
       //println(s"recording $str, $isType, $entry")
 
       /** If `e1` is an alias of another class of the same name, return the other
@@ -146,7 +145,7 @@ object Message:
       }
 
       def addendum(cat: String, info: Type): String = info match {
-        case bounds @ TypeBounds(lo, hi) if !(bounds =:= TypeBounds.empty) =>
+        case bounds @ TypeBounds(lo, hi) if !(bounds =:= TypeBounds.empty) && !bounds.isErroneous =>
           if (lo eq hi) i" which is an alias of $lo"
           else i" with $cat ${boundsStr(bounds)}"
         case _ =>
@@ -176,9 +175,8 @@ object Message:
       def needsExplanation(entry: Recorded) = entry match {
         case param: TypeParamRef => ctx.typerState.constraint.contains(param)
         case param: ParamRef     => false
-        case skolem: SkolemType => true
-        case sym: Symbol =>
-          ctx.gadt.contains(sym) && ctx.gadt.fullBounds(sym) != TypeBounds.empty
+        case skolem: SkolemType  => true
+        case sym: Symbol         => ctx.gadt.contains(sym) && ctx.gadt.fullBounds(sym) != TypeBounds.empty
       }
 
       val toExplain: List[(String, Recorded)] = seen.toList.flatMap { kvs =>
@@ -191,7 +189,7 @@ object Message:
               (tickedString, alt)
             }
         }
-        res // help the inferrencer out
+        res // help the inferencer out
       }.sortBy(_._1)
 
       def columnar(parts: List[(String, String)]): List[String] = {
@@ -270,11 +268,11 @@ end Message
   *
   * Messages modify the rendendering of interpolated strings in several ways:
   *
-  *  1. The size of the printed code is limited with a MessafeLimiter. If the message
+  *  1. The size of the printed code is limited with a MessageLimiter. If the message
   *    would get too large or too deeply nested, a `...` is printed instead.
-  *  2. References to module classes are prefixed with `object ` for better recogniability.
+  *  2. References to module classes are prefixed with `object` for better recognizability.
   *  3. A where clause is sometimes added which contains the following additional explanations:
-  *     - Rerences are disambiguated: If a message contains occurrences of the same identifier
+  *     - References are disambiguated: If a message contains occurrences of the same identifier
   *       representing different symbols, the duplicates are printed with superscripts
   *       and the where-clause explains where each symbol is located.
   *     - Uninstantiated variables are explained in the where-clause with additional
diff --git a/tests/neg/i19334.check b/tests/neg/i19334.check
new file mode 100644
index 000000000000..8c0b6b3c299e
--- /dev/null
+++ b/tests/neg/i19334.check
@@ -0,0 +1,12 @@
+-- [E081] Type Error: tests/neg/i19334.scala:6:4 -----------------------------------------------------------------------
+6 |  f(_) // error was OOM formatting TypeVar(TypeParamRef(T)) when offering explanations
+  |    ^
+  |    Missing parameter type
+  |
+  |    I could not infer the type of the parameter _$1
+  |    in expanded function:
+  |      _$1 => f(_$1)
+  |    Expected type for the whole anonymous function:
+  |      T
+  |
+  |    where:    T is a type variable
diff --git a/tests/neg/i19334.scala b/tests/neg/i19334.scala
new file mode 100644
index 000000000000..cd6af1872241
--- /dev/null
+++ b/tests/neg/i19334.scala
@@ -0,0 +1,6 @@
+
+def foo[T](f: T): T = ???
+
+@main def main = foo:
+  def f() = ()
+  f(_) // error was OOM formatting TypeVar(TypeParamRef(T)) when offering explanations