[ConstraintSystem] Don't attempt to optimize disjunction in diagnostic mode

Don't attempt disjunction optimization in "diagnostic mode"
because in such mode we'd like to attempt all of the available
overloads regardless of of problems related to missing or
extraneous labels and/or arguments.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6072,14 +6072,20 @@ ConstraintSystem::simplifyApplicableFnConstraint(
 
   };
 
-  // If the right-hand side is a type variable, try to simplify the overload
-  // set.
-  if (auto typeVar = desugar2->getAs<TypeVariableType>()) {
-    Type newType2 = simplifyAppliedOverloads(typeVar, func1, locator);
-    if (!newType2)
-      return SolutionKind::Error;
+  // Don't attempt this optimization in "diagnostic mode" because
+  // in such mode we'd like to attempt all of the available
+  // overloads regardless of of problems related to missing or
+  // extraneous labels and/or arguments.
+  if (!(solverState && shouldAttemptFixes())) {
+    // If the right-hand side is a type variable,
+    // try to simplify the overload set.
+    if (auto typeVar = desugar2->getAs<TypeVariableType>()) {
+      Type newType2 = simplifyAppliedOverloads(typeVar, func1, locator);
+      if (!newType2)
+        return SolutionKind::Error;
 
-    desugar2 = newType2->getDesugaredType();
+      desugar2 = newType2->getDesugaredType();
+    }
   }
 
   // If right-hand side is a type variable, the constraint is unsolved.