Fix for SI-6367, exponential time in inference.

This pathology is not new - it can be witnessed in 2.9,
where compiling the test case enclosed with this ticket with
-Yinfer-debug will print a line with (pinky to lips) one million
type parameters. 1048576 actually, aka 2^20. But in 2.9 we were
somehow getting away with creating the list, presumably by not
spending much time looking at it. Somewhere between there and M1,
that changed.

I cut it off at the knees - don't create a list of one million
upper bound constraints when 1 will suffice. It would not be too
surprising if this proves to be a boon for performance.

1 files changed, 17 insertions, 13 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index df44cf234e..29f7b53154 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -3935,13 +3935,15 @@ trait Types extends api.Types { self: SymbolTable =>
     def avoidWiden: Boolean = avoidWidening
 
     def addLoBound(tp: Type, isNumericBound: Boolean = false) {
-      if (isNumericBound && isNumericValueType(tp)) {
-        if (numlo == NoType || isNumericSubType(numlo, tp))
-          numlo = tp
-        else if (!isNumericSubType(tp, numlo))
-          numlo = numericLoBound
+      if (!lobounds.contains(tp)) {
+        if (isNumericBound && isNumericValueType(tp)) {
+          if (numlo == NoType || isNumericSubType(numlo, tp))
+            numlo = tp
+          else if (!isNumericSubType(tp, numlo))
+            numlo = numericLoBound
+        }
+        else lobounds ::= tp
       }
-      else lobounds ::= tp
     }
 
     def checkWidening(tp: Type) {
@@ -3953,14 +3955,16 @@ trait Types extends api.Types { self: SymbolTable =>
     }
 
     def addHiBound(tp: Type, isNumericBound: Boolean = false) {
-      checkWidening(tp)
-      if (isNumericBound && isNumericValueType(tp)) {
-        if (numhi == NoType || isNumericSubType(tp, numhi))
-          numhi = tp
-        else if (!isNumericSubType(numhi, tp))
-          numhi = numericHiBound
+      if (!hibounds.contains(tp)) {
+        checkWidening(tp)
+        if (isNumericBound && isNumericValueType(tp)) {
+          if (numhi == NoType || isNumericSubType(tp, numhi))
+            numhi = tp
+          else if (!isNumericSubType(numhi, tp))
+            numhi = numericHiBound
+        }
+        else hibounds ::= tp
       }
-      else hibounds ::= tp
     }
 
     def isWithinBounds(tp: Type): Boolean =