diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp --- a/clang/lib/Sema/SemaTemplate.cpp +++ b/clang/lib/Sema/SemaTemplate.cpp @@ -2494,6 +2494,12 @@ if (!CD || (!FTD && CD->isFunctionTemplateSpecialization())) continue; + // Cannot make a deduction guide when unparsed arguments are present. + if (std::any_of(CD->param_begin(), CD->param_end(), [](ParmVarDecl *P) { + return !P || P->hasUnparsedDefaultArg(); + })) + continue; + Transform.transformConstructor(FTD, CD); AddedAny = true; } diff --git a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp --- a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp +++ b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp @@ -416,6 +416,17 @@ } +namespace no_crash_on_default_arg { +class A { + template class B { + B(int c = 1); + }; + // This used to crash due to unparsed default arg above. The diagnostic could + // be improved, but the point of this test is to simply check we do not crash. + B(); // expected-error {{deduction guide declaration without trailing return type}} +}; +} // namespace no_crash_on_default_arg + #pragma clang diagnostic push #pragma clang diagnostic warning "-Wctad-maybe-unsupported" namespace test_implicit_ctad_warning {