[Clang] Fix a crash when diagnosing wrong conversion to explicit object parameter

When an overload is invalid, we try to initialize each conversion sequence
for the purpose of recovery, but we failed to initialized explicit objects,
leading to a crash

Fixes #147121

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -930,6 +930,7 @@ Bug Fixes to C++ Support
 - Fix a bug where private access specifier of overloaded function not respected. (#GH107629)
 - Correctly handles calling an explicit object member function template overload set
   through its address (``(&Foo::bar<baz>)()``).
+- Fix a crash when forming an invalid call to an operator with an explicit object member. (#GH147121)
 - Correctly handle allocations in the condition of a ``if constexpr``.(#GH120197) (#GH134820)
 - Fixed a crash when handling invalid member using-declaration in C++20+ mode. (#GH63254)
 - Fixed parsing of lambda expressions that appear after ``*`` or ``&`` in contexts where a declaration can appear. (#GH63880)

clang/lib/Sema/SemaOverload.cpp

@@ -13131,7 +13131,8 @@ CompleteNonViableCandidate(Sema &S, OverloadCandidate *Cand,
     ParamTypes =
         Cand->Function->getType()->castAs<FunctionProtoType>()->getParamTypes();
     if (isa<CXXMethodDecl>(Cand->Function) &&
-        !isa<CXXConstructorDecl>(Cand->Function) && !Reversed) {
+        !isa<CXXConstructorDecl>(Cand->Function) && !Reversed &&
+        !Cand->Function->hasCXXExplicitFunctionObjectParameter()) {
       // Conversion 0 is 'this', which doesn't have a corresponding parameter.
       ConvIdx = 1;
       if (CSK == OverloadCandidateSet::CSK_Operator &&
@@ -13149,9 +13150,8 @@ CompleteNonViableCandidate(Sema &S, OverloadCandidate *Cand,
 
   // Fill in the rest of the conversions.
   for (unsigned ParamIdx = Reversed ? ParamTypes.size() - 1 : 0;
-       ConvIdx != ConvCount;
+       ConvIdx != ConvCount && ArgIdx < Args.size();
        ++ConvIdx, ++ArgIdx, ParamIdx += (Reversed ? -1 : 1)) {
-    assert(ArgIdx < Args.size() && "no argument for this arg conversion");
     if (Cand->Conversions[ConvIdx].isInitialized()) {
       // We've already checked this conversion.
     } else if (ParamIdx < ParamTypes.size()) {

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

@@ -1290,3 +1290,60 @@ void f() {
 
 
 }
+
+namespace GH147121 {
+struct X {};
+struct S1 {
+    bool operator==(this auto &&, const X &); // #S1-cand
+};
+struct S2 {
+    bool operator==(this X, const auto &&); // #S2-cand
+};
+
+struct S3 {
+    S3& operator++(this X); // #S3-inc-cand
+    S3& operator++(this int); // #S3-inc-cand
+    int operator[](this X); // #S3-sub-cand
+    int operator[](this int); // #S3-sub-cand2
+    void f(this X); // #S3-f-cand
+    void f(this int); // #S3-f-cand2
+};
+
+int main() {
+    S1{} == S1{};
+    // expected-error@-1 {{invalid operands to binary expression ('S1' and 'S1')}}
+    // expected-note@#S1-cand {{candidate function template not viable}}
+    // expected-note@#S1-cand {{candidate function (with reversed parameter order) template not viable}}
+
+
+    S1{} != S1{};
+    // expected-error@-1 {{invalid operands to binary expression ('S1' and 'S1')}}
+    // expected-note@#S1-cand {{candidate function template not viable}}
+    // expected-note@#S1-cand {{candidate function (with reversed parameter order) template not viable}}
+
+
+    S2{} == S2{};
+    // expected-error@-1 {{invalid operands to binary expression ('S2' and 'S2')}}
+    // expected-note@#S2-cand {{candidate function template not viable}}
+    // expected-note@#S2-cand {{candidate function (with reversed parameter order) template not viable}}
+
+
+    S2{} != S2{};
+    // expected-error@-1 {{invalid operands to binary expression ('S2' and 'S2')}}
+    // expected-note@#S2-cand {{candidate function template not viable}}
+    // expected-note@#S2-cand {{candidate function (with reversed parameter order) template not viable}}
+
+    S3 s3;
+    ++s3;
+    // expected-error@-1{{cannot increment value of type 'S3'}}
+    s3[];
+    // expected-error@-1{{no viable overloaded operator[] for type 'S3'}}
+    // expected-note@#S3-sub-cand {{candidate function not viable: no known conversion from 'S3' to 'X' for object argument}}
+    // expected-note@#S3-sub-cand2 {{candidate function not viable: no known conversion from 'S3' to 'int' for object argument}}
+
+    s3.f();
+    // expected-error@-1{{no matching member function for call to 'f'}}
+    // expected-note@#S3-f-cand {{candidate function not viable: no known conversion from 'S3' to 'X' for object argument}}
+    // expected-note@#S3-f-cand2 {{candidate function not viable: no known conversion from 'S3' to 'int' for object argument}}
+}
+}