[Clang] prevent null explicit object argument from being deduced (#104328)

Fixes #102025

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -257,6 +257,7 @@ Bug Fixes to C++ Support
 - Properly reject defaulted relational operators with invalid types for explicit object parameters,
   e.g., ``bool operator==(this int, const Foo&)`` (#GH100329), and rvalue reference parameters.
 - Properly reject defaulted copy/move assignment operators that have a non-reference explicit object parameter.
+- Fixed an assertion failure by preventing null explicit object arguments from being deduced. (#GH102025).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -4462,6 +4462,9 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
       ParamTypesForArgChecking.push_back(ParamType);
 
       if (ParamIdx == 0 && HasExplicitObject) {
+        if (ObjectType.isNull())
+          return TemplateDeductionResult::InvalidExplicitArguments;
+
         if (auto Result = DeduceCallArgument(ParamType, 0,
                                              /*ExplicitObjectArgument=*/true);
             Result != TemplateDeductionResult::Success)

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -1011,3 +1011,22 @@ struct X;
 static_assert(__is_same(X<B{0}>, X<B{0}>));
 static_assert(!__is_same(X<B{0}>, X<B{1}>));
 } // namespace defaulted_compare
+
+namespace GH102025 {
+struct Foo {
+  template <class T>
+  constexpr auto operator[](this T &&self, auto... i) // expected-note {{candidate template ignored: substitution failure [with T = Foo &, i:auto = <>]: member '_evaluate' used before its declaration}}
+      -> decltype(_evaluate(self, i...)) {
+    return self._evaluate(i...);
+  }
+
+private:
+  template <class T>
+  constexpr auto _evaluate(this T &&self, auto... i) -> decltype((i + ...));
+};
+
+int main() {
+  Foo foo;
+  return foo[]; // expected-error {{no viable overloaded operator[] for type 'Foo'}}
+}
+}