[ConstraintSystem] Deduplicate generic requirements after merging type variables

This comes up, for example, when `Self` type parameter is bound to its
argument in a member reference, both argument and parameter can refer
to exact same requirements, that have to be deduplicated otherwise
they are just going to get "fixed" N times, which complicates diagnostics.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -126,6 +126,33 @@ void ConstraintSystem::mergeEquivalenceClasses(TypeVariableType *typeVar1,
   // Merge nodes in the constraint graph.
   CG.mergeNodes(typeVar1, typeVar2);
 
+  // Let's try to de-duplicate any generic requirements
+  // associated with given type variable, if it represents
+  // a generic parameter.
+  if (typeVar1->getImpl().getGenericParameter()) {
+    auto requirements = CG.gatherConstraints(
+        typeVar1, ConstraintGraph::GatheringKind::EquivalenceClass,
+        [](Constraint *constraint) -> bool {
+          if (auto *locator = constraint->getLocator()) {
+            return locator->isLastElement(
+                       ConstraintLocator::TypeParameterRequirement) ||
+                   locator->isLastElement(
+                       ConstraintLocator::ConditionalRequirement);
+          }
+
+          return false;
+        });
+
+    llvm::SmallPtrSet<TypeBase *, 4> existingRequirements;
+    for (auto *req : requirements) {
+      auto constraintTy = req->getSecondType();
+      // If this constraint has already been recorded in the
+      // constraint system, there is no reason to check it twice.
+      if (!existingRequirements.insert(constraintTy.getPointer()).second)
+        retireConstraint(req);
+    }
+  }
+
   if (updateWorkList) {
     addTypeVariableConstraintsToWorkList(typeVar1);
   }