[CSFix] Implement coalescing for generic argument mismatch fixes

Author: xedin

include/swift/Sema/CSFix.h

@@ -1100,6 +1100,10 @@ class GenericArgumentsMismatch final
     return {getTrailingObjects<unsigned>(), NumMismatches};
   }
 
+  bool coalesceAndDiagnose(const Solution &solution,
+                           ArrayRef<ConstraintFix *> secondaryFixes,
+                           bool asNote = false) const override;
+
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
   static GenericArgumentsMismatch *create(ConstraintSystem &cs, Type actual,
@@ -1112,6 +1116,9 @@ class GenericArgumentsMismatch final
   }
 
 private:
+  bool diagnose(const Solution &solution, ArrayRef<unsigned> mismatches,
+                bool asNote = false) const;
+
   MutableArrayRef<unsigned> getMismatchesBuf() {
     return {getTrailingObjects<unsigned>(), NumMismatches};
   }

lib/Sema/CSFix.cpp

@@ -732,10 +732,31 @@ AllowFunctionTypeMismatch::create(ConstraintSystem &cs, Type lhs, Type rhs,
       AllowFunctionTypeMismatch(cs, lhs, rhs, locator, index);
 }
 
+bool GenericArgumentsMismatch::coalesceAndDiagnose(
+    const Solution &solution, ArrayRef<ConstraintFix *> secondaryFixes,
+    bool asNote) const {
+  std::set<unsigned> scratch(getMismatches().begin(), getMismatches().end());
+
+  for (auto *fix : secondaryFixes) {
+    auto *genericArgsFix = fix->castTo<GenericArgumentsMismatch>();
+    for (auto mismatchIdx : genericArgsFix->getMismatches())
+      scratch.insert(mismatchIdx);
+  }
+
+  SmallVector<unsigned> mismatches(scratch.begin(), scratch.end());
+  return diagnose(solution, mismatches, asNote);
+}
+
+bool GenericArgumentsMismatch::diagnose(const Solution &solution,
+                                        bool asNote) const {
+  return diagnose(solution, getMismatches(), asNote);
+}
+
 bool GenericArgumentsMismatch::diagnose(const Solution &solution,
+                                        ArrayRef<unsigned> mismatches,
                                         bool asNote) const {
   GenericArgumentsMismatchFailure failure(solution, getFromType(), getToType(),
-                                          getMismatches(), getLocator());
+                                          mismatches, getLocator());
   return failure.diagnose(asNote);
 }