Sema: Try not to make a negative impact

Author: slavapestov

include/swift/Sema/ConstraintSystem.h

@@ -1045,13 +1045,15 @@ struct Score {
   friend Score operator-(const Score &x, const Score &y) {
     Score result;
     for (unsigned i = 0; i != NumScoreKinds; ++i) {
+      ASSERT(x.Data[i] >= y.Data[i]);
       result.Data[i] = x.Data[i] - y.Data[i];
     }
     return result;
   }
 
   friend Score &operator-=(Score &x, const Score &y) {
     for (unsigned i = 0; i != NumScoreKinds; ++i) {
+      ASSERT(x.Data[i] >= y.Data[i]);
       x.Data[i] -= y.Data[i];
     }
     return x;

lib/Sema/CSSimplify.cpp

@@ -15032,9 +15032,12 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     SmallVector<Type, 4> unwraps2;
     type2->lookThroughAllOptionalTypes(unwraps2);
 
-    auto impact = unwraps1.size() != unwraps2.size()
-                      ? unwraps1.size() - unwraps2.size()
-                      : 1;
+    unsigned impact = 1;
+    if (unwraps1.size() > unwraps2.size())
+      impact = unwraps1.size() - unwraps2.size();
+    else if (unwraps2.size() > unwraps1.size())
+      impact = unwraps2.size() - unwraps1.size();
+
     return recordFix(fix, impact) ? SolutionKind::Error : SolutionKind::Solved;
   }