SI-8158 compiler hangs printing out fancy types

Apparently, even though the compiler has safeguards against infinite type
printouts, having a depth counter, we didn’t account for the cases when
printouts are both self-referential and self-multiplying.

For one, SI-8158 provides an example of such a type, which is a structural
type that refers to itself twice in return types of its methods. At first,
printing such a type would go deeper and deeper, but then it will hit
the depth limit and start multiply indefinitely.

This commit fixes this particular problem by recognizing self-references
as this.type’s and printing them out as such. The subsequent commit will
introduce a more general facility.

1 files changed, 9 insertions, 4 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index c2689fe7e9..54e5b2781e 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -2519,6 +2519,10 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
         if (settings.debug.value) parentsString(tp.parents)
         else briefParentsString(tp.parents)
       )
+      def isStructuralThisType = (
+        // prevents disasters like SI-8158
+        owner.isInitialized && owner.isStructuralRefinement && tp == owner.tpe
+      )
       if (isType) typeParamsString(tp) + (
         if (isClass) " extends " + parents
         else if (isAliasType) " = " + tp.resultType
@@ -2529,10 +2533,11 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
       )
       else if (isModule) "" //  avoid "object X of type X.type"
       else tp match {
-        case PolyType(tparams, res)  => typeParamsString(tp) + infoString(res)
-        case NullaryMethodType(res)  => infoString(res)
-        case MethodType(params, res) => valueParamsString(tp) + infoString(res)
-        case _                       => ": " + tp
+        case PolyType(tparams, res)    => typeParamsString(tp) + infoString(res)
+        case NullaryMethodType(res)    => infoString(res)
+        case MethodType(params, res)   => valueParamsString(tp) + infoString(res)
+        case _ if isStructuralThisType => ": this.type"
+        case _                         => ": " + tp
       }
     }