[CSFix] Add a fix for invalid reference to mutating member on immutable base

Author: xedin

lib/Sema/CSFix.cpp

@@ -643,3 +643,15 @@ bool SkipUnhandledConstructInFunctionBuilder::diagnose(Expr *root,
       root, getConstraintSystem(), unhandled, builder, getLocator());
   return failure.diagnose(asNote);
 }
+
+bool AllowMutatingMemberOrRValueBase::diagnose(Expr *root, bool asNote) const {
+  return false;
+}
+
+AllowMutatingMemberOrRValueBase *
+AllowMutatingMemberOrRValueBase::create(ConstraintSystem &cs, Type baseType,
+                                        ValueDecl *member, DeclName name,
+                                        ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      AllowMutatingMemberOrRValueBase(cs, baseType, member, name, locator);
+}

lib/Sema/CSFix.h

@@ -179,6 +179,10 @@ enum class FixKind : uint8_t {
   /// matches up with a
   /// parameter that has a function builder.
   SkipUnhandledConstructInFunctionBuilder,
+
+  /// Allow invalid reference to a member declared as `mutating`
+  /// when base is an r-value type.
+  AllowMutatingMemberOrRValueBase,
 };
 
 class ConstraintFix {
@@ -858,6 +862,25 @@ class AllowInvalidInitRef final : public ConstraintFix {
                                      ConstraintLocator *locator);
 };
 
+class AllowMutatingMemberOrRValueBase final : public AllowInvalidMemberRef {
+  AllowMutatingMemberOrRValueBase(ConstraintSystem &cs, Type baseType,
+                                  ValueDecl *member, DeclName name,
+                                  ConstraintLocator *locator)
+      : AllowInvalidMemberRef(cs, FixKind::AllowMutatingMemberOrRValueBase,
+                              baseType, member, name, locator) {}
+
+public:
+  std::string getName() const override {
+    return "allow `mutating` method on r-value base";
+  }
+
+  bool diagnose(Expr *root, bool asNote = false) const override;
+
+  static AllowMutatingMemberOrRValueBase *
+  create(ConstraintSystem &cs, Type baseType, ValueDecl *member, DeclName name,
+         ConstraintLocator *locator);
+};
+
 class AllowClosureParamDestructuring final : public ConstraintFix {
   FunctionType *ContextualType;
 

lib/Sema/CSSimplify.cpp

@@ -4761,7 +4761,13 @@ fixMemberRef(ConstraintSystem &cs, Type baseTy,
     }
 
     case MemberLookupResult::UR_MutatingMemberOnRValue:
-    case MemberLookupResult::UR_MutatingGetterOnRValue:
+    case MemberLookupResult::UR_MutatingGetterOnRValue: {
+      return choice.isDecl()
+                 ? AllowMutatingMemberOrRValueBase::create(
+                       cs, baseTy, choice.getDecl(), memberName, locator)
+                 : nullptr;
+    }
+
     case MemberLookupResult::UR_LabelMismatch:
     // TODO(diagnostics): Add a new fix that is suggests to
     // add `subscript(dynamicMember: {Writable}KeyPath<T, U>)`
@@ -6929,6 +6935,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowInvalidRefInKeyPath:
   case FixKind::ExplicitlySpecifyGenericArguments:
   case FixKind::GenericArgumentsMismatch:
+  case FixKind::AllowMutatingMemberOrRValueBase:
     llvm_unreachable("handled elsewhere");
   }