[CSFix] Allow diagnosing invalid references in key path with multiple solutions

If multiple solutions have exactly a fix for exactly the same invalid member
reference in key path, let's diagnose that as if there is no ambiguity there.

Author: xedin

include/swift/Sema/CSFix.h

@@ -1520,10 +1520,18 @@ class AllowInvalidRefInKeyPath final : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
+
   /// Determine whether give reference requires a fix and produce one.
   static AllowInvalidRefInKeyPath *
   forRef(ConstraintSystem &cs, ValueDecl *member, ConstraintLocator *locator);
 
+  bool isEqual(const ConstraintFix *other) const;
+
+  static bool classof(const ConstraintFix *fix) {
+    return fix->getKind() == FixKind::AllowInvalidRefInKeyPath;
+  }
+
 private:
   static AllowInvalidRefInKeyPath *create(ConstraintSystem &cs, RefKind kind,
                                           ValueDecl *member,

lib/Sema/CSFix.cpp

@@ -953,6 +953,29 @@ bool AllowInvalidRefInKeyPath::diagnose(const Solution &solution,
   llvm_unreachable("covered switch");
 }
 
+bool AllowInvalidRefInKeyPath::diagnoseForAmbiguity(
+    CommonFixesArray commonFixes) const {
+  auto *primaryFix =
+      commonFixes.front().second->getAs<AllowInvalidRefInKeyPath>();
+  assert(primaryFix);
+
+  if (llvm::all_of(
+          commonFixes,
+          [&primaryFix](
+              const std::pair<const Solution *, const ConstraintFix *> &entry) {
+            return primaryFix->isEqual(entry.second);
+          })) {
+    return diagnose(*commonFixes.front().first);
+  }
+
+  return false;
+}
+
+bool AllowInvalidRefInKeyPath::isEqual(const ConstraintFix *other) const {
+  auto *refFix = other->getAs<AllowInvalidRefInKeyPath>();
+  return refFix ? Kind == refFix->Kind && Member == refFix->Member : false;
+}
+
 AllowInvalidRefInKeyPath *
 AllowInvalidRefInKeyPath::forRef(ConstraintSystem &cs, ValueDecl *member,
                                  ConstraintLocator *locator) {