llvm/clang/test/SemaCXX/invalid-deduction-guide-as-template-candidates.cpp

// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s

template <class T> class Foo {}; // expected-note {{candidate template ignored: couldn't infer template argument 'T'}} \
                                 // expected-note {{implicit deduction guide declared as 'template <class T> Foo(Foo<T>) -> Foo<T>'}} \
                                 // expected-note {{candidate function template not viable: requires 1 argument, but 0 were provided}} \
                                 // expected-note {{implicit deduction guide declared as 'template <class T> Foo() -> Foo<T>'}}
Foo(); // expected-error {{deduction guide declaration without trailing return type}}
Foo vs; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'Foo'}}