clang/test/SemaTemplate/deduction.cpp

   1 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
   2 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++1z
   3 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++2b
   4
   5 // Template argument deduction with template template parameters.
   6 template<typename T, template<T> class A>
   7 struct X0 {
   8   static const unsigned value = 0;
   9 };
  10
  11 template<template<int> class A>
  12 struct X0<int, A> {
  13   static const unsigned value = 1;
  14 };
  15
  16 template<int> struct X0i;
  17 template<long> struct X0l;
  18 int array_x0a[X0<long, X0l>::value == 0? 1 : -1];
  19 int array_x0b[X0<int, X0i>::value == 1? 1 : -1];
  20
  21 template<typename T, typename U>
  22 struct is_same {
  23   static const bool value = false;
  24 };
  25
  26 template<typename T>
  27 struct is_same<T, T> {
  28   static const bool value = true;
  29 };
  30
  31 template<typename T> struct allocator { };
  32 template<typename T, typename Alloc = allocator<T> > struct vector {};
  33
  34 // Fun with meta-lambdas!
  35 struct _1 {};
  36 struct _2 {};
  37
  38 // Replaces all occurrences of _1 with Arg1 and _2 with Arg2 in T.
  39 template<typename T, typename Arg1, typename Arg2>
  40 struct Replace {
  41   typedef T type;
  42 };
  43
  44 // Replacement of the whole type.
  45 template<typename Arg1, typename Arg2>
  46 struct Replace<_1, Arg1, Arg2> {
  47   typedef Arg1 type;
  48 };
  49
  50 template<typename Arg1, typename Arg2>
  51 struct Replace<_2, Arg1, Arg2> {
  52   typedef Arg2 type;
  53 };
  54
  55 // Replacement through cv-qualifiers
  56 template<typename T, typename Arg1, typename Arg2>
  57 struct Replace<const T, Arg1, Arg2> {
  58   typedef typename Replace<T, Arg1, Arg2>::type const type;
  59 };
  60
  61 // Replacement of templates
  62 template<template<typename> class TT, typename T1, typename Arg1, typename Arg2>
  63 struct Replace<TT<T1>, Arg1, Arg2> {
  64   typedef TT<typename Replace<T1, Arg1, Arg2>::type> type;
  65 };
  66
  67 template<template<typename, typename> class TT, typename T1, typename T2,
  68          typename Arg1, typename Arg2>
  69 struct Replace<TT<T1, T2>, Arg1, Arg2> {
  70   typedef TT<typename Replace<T1, Arg1, Arg2>::type,
  71              typename Replace<T2, Arg1, Arg2>::type> type;
  72 };
  73
  74 // Just for kicks...
  75 template<template<typename, typename> class TT, typename T1,
  76          typename Arg1, typename Arg2>
  77 struct Replace<TT<T1, _2>, Arg1, Arg2> {
  78   typedef TT<typename Replace<T1, Arg1, Arg2>::type, Arg2> type;
  79 };
  80
  81 int array0[is_same<Replace<_1, int, float>::type, int>::value? 1 : -1];
  82 int array1[is_same<Replace<const _1, int, float>::type, const int>::value? 1 : -1];
  83 int array2[is_same<Replace<vector<_1>, int, float>::type, vector<int> >::value? 1 : -1];
  84 int array3[is_same<Replace<vector<const _1>, int, float>::type, vector<const int> >::value? 1 : -1];
  85 int array4[is_same<Replace<vector<int, _2>, double, float>::type, vector<int, float> >::value? 1 : -1];
  86
  87 // PR5911
  88 template <typename T, int N> void f(const T (&a)[N]);
  89 int iarr[] = { 1 };
  90 void test_PR5911() { f(iarr); }
  91
  92 // Must not examine base classes of incomplete type during template argument
  93 // deduction.
  94 namespace PR6257 {
  95   template <typename T> struct X {
  96     template <typename U> X(const X<U>& u);
  97   };
  98   struct A;
  99   void f(A& a);
 100   void f(const X<A>& a);
 101   void test(A& a) { (void)f(a); }
 102 }
 103
 104 // PR7463
 105 namespace PR7463 {
 106   const int f ();
 107   template <typename T_> void g (T_&); // expected-note{{T_ = int}}
 108   void h (void) { g(f()); } // expected-error{{no matching function for call}}
 109 }
 110
 111 namespace test0 {
 112   template <class T> void make(const T *(*fn)()); // expected-note {{candidate template ignored: cannot deduce a type for 'T' that would make 'const T' equal 'char'}}
 113   char *char_maker();
 114   void test() {
 115     make(char_maker); // expected-error {{no matching function for call to 'make'}}
 116   }
 117 }
 118
 119 namespace test1 {
 120   template<typename T> void foo(const T a[3][3]);
 121   void test() {
 122     int a[3][3];
 123     foo(a);
 124   }
 125 }
 126
 127 // PR7708
 128 namespace test2 {
 129   template<typename T> struct Const { typedef void const type; };
 130
 131   template<typename T> void f(T, typename Const<T>::type*);
 132   template<typename T> void f(T, void const *);
 133
 134   void test() {
 135     void *p = 0;
 136     f(0, p);
 137   }
 138 }
 139
 140 // rdar://problem/8537391
 141 namespace test3 {
 142   struct Foo {
 143     template <void F(char)> static inline void foo();
 144   };
 145
 146   class Bar {
 147     template<typename T> static inline void wobble(T ch);
 148
 149   public:
 150     static void madness() {
 151       Foo::foo<wobble<char> >();
 152     }
 153   };
 154 }
 155
 156 namespace test4 {
 157
 158 template <class> struct a { using b = const float; };
 159 template <class c> using d = typename a<c>::b;
 160
 161 template <class c> void e(d<c> *, c) {}
 162 template void e(const float *, int);
 163
 164 } // namespace test4
 165
 166 namespace test5 {
 167
 168 template <bool, int = 0> class a {};
 169 template <class b> void c(b, b);
 170 template <bool b> void c(a<b>, a<b>);
 171 void d() { c(a<true>(), a<true>()); }
 172
 173 } // namespace test5
 174
 175 namespace test6 {
 176 template <class A1> using A = A1;
 177
 178 template <class F1, class... F2> void f(A<F1>, F1, F2...);
 179 template <class F3> void f(A<F3>, F3);
 180
 181 void g() { f(A<int>{}, int{}); }
 182 } // namespace test6
 183
 184 namespace test7 {
 185 template <class T> void f(T&, T&);
 186 template <class T, unsigned long S> void f(T (&)[S], T (&)[S]);
 187
 188 void g() {
 189   int i[3], j[3];
 190   f(i, j);
 191 }
 192 } // namespace test7
 193
 194 namespace test8 {
 195 template <class T> void foo(T);
 196 void test(int a) {
 197     char n[a];
 198     foo(n);
 199 }
 200 } // namespace test8
 201
 202 // Verify that we can deduce enum-typed arguments correctly.
 203 namespace test14 {
 204   enum E { E0, E1 };
 205   template <E> struct A {};
 206   template <E e> void foo(const A<e> &a) {}
 207
 208   void test() {
 209     A<E0> a;
 210     foo(a);
 211   }
 212 }
 213
 214 namespace PR21536 {
 215   template<typename ...T> struct X;
 216   template<typename A, typename ...B> struct S {
 217     static_assert(sizeof...(B) == 1, "");
 218     void f() {
 219       using T = A;
 220       using T = int;
 221
 222       using U = X<B...>;
 223       using U = X<int>;
 224     }
 225   };
 226   template<typename ...T> void f(S<T...>);
 227   void g() { f(S<int, int>()); }
 228 }
 229
 230 namespace PR19372 {
 231   template <template<typename...> class C, typename ...Us> struct BindBack {
 232     template <typename ...Ts> using apply = C<Ts..., Us...>;
 233   };
 234   template <typename, typename...> struct Y;
 235   template <typename ...Ts> using Z = Y<Ts...>;
 236
 237   using T = BindBack<Z, int>::apply<>;
 238   using T = Z<int>;
 239
 240   using U = BindBack<Z, int, int>::apply<char>;
 241   using U = Z<char, int, int>;
 242
 243   namespace BetterReduction {
 244     template<typename ...> struct S;
 245     template<typename ...A> using X = S<A...>; // expected-note {{parameter}}
 246     template<typename ...A> using Y = X<A..., A...>;
 247     template<typename ...A> using Z = X<A..., 1, 2, 3>; // expected-error {{must be a type}}
 248
 249     using T = Y<int>;
 250     using T = S<int, int>;
 251   }
 252 }
 253
 254 namespace PR18645 {
 255   template<typename F> F Quux(F &&f);
 256   auto Baz = Quux(Quux<float>);
 257 }
 258
 259 namespace NonDeducedNestedNameSpecifier {
 260   template<typename T> struct A {
 261     template<typename U> struct B {
 262       B(int) {}
 263     };
 264   };
 265
 266   template<typename T> int f(A<T>, typename A<T>::template B<T>);
 267   int k = f(A<int>(), 0);
 268 }
 269
 270 namespace PR27601_RecursivelyInheritedBaseSpecializationsDeductionAmbiguity {
 271 namespace ns1 {
 272
 273 template<class...> struct B { };
 274 template<class H, class ... Ts> struct B<H, Ts...> : B<> { };
 275 template<class ... Ts> struct D : B<Ts...> { };
 276
 277 template<class T, class ... Ts> void f(B<T, Ts...> &) { }
 278
 279 int main() {
 280   D<int, char> d;
 281   f<int>(d);
 282 }
 283 } //end ns1
 284
 285 namespace ns2 {
 286
 287 template <int i, typename... Es> struct tup_impl;
 288
 289 template <int i> struct tup_impl<i> {}; // empty tail
 290
 291 template <int i, typename Head, typename... Tail>
 292 struct tup_impl<i, Head, Tail...> : tup_impl<i + 1, Tail...> {
 293   using value_type = Head;
 294   Head head;
 295 };
 296
 297 template <typename... Es> struct tup : tup_impl<0, Es...> {};
 298
 299 template <typename Head, int i, typename... Tail>
 300 Head &get_helper(tup_impl<i, Head, Tail...> &t) {
 301   return t.head;
 302 }
 303
 304 template <typename Head, int i, typename... Tail>
 305 Head const &get_helper(tup_impl<i, Head, Tail...> const &t) {
 306   return t.head;
 307 }
 308
 309 int main() {
 310   tup<int, double, char> t;
 311   get_helper<double>(t);
 312   return 0;
 313 }
 314 } // end ns2
 315 }
 316
 317 namespace multiple_deduction_different_type {
 318   template<typename T, T v> struct X {};
 319   template<template<typename T, T> class X, typename T, typename U, int N>
 320     void f(X<T, N>, X<U, N>) {} // expected-note 2{{values of conflicting types}}
 321   template<template<typename T, T> class X, typename T, typename U, const int *N>
 322     void g(X<T, N>, X<U, N>) {} // expected-note 0-2{{values of conflicting types}}
 323   int n;
 324   void h() {
 325     f(X<int, 1+1>(), X<unsigned int, 3-1>()); // expected-error {{no matching function}}
 326     f(X<unsigned int, 1+1>(), X<int, 3-1>()); // expected-error {{no matching function}}
 327 #if __cplusplus > 201402L
 328     g(X<const int*, &n>(), X<int*, &n + 1 - 1>()); // expected-error {{no matching function}}
 329     g(X<int*, &n>(), X<const int*, &n + 1 - 1>()); // expected-error {{no matching function}}
 330 #endif
 331   }
 332
 333   template<template<typename T, T> class X, typename T, typename U, T N>
 334     void x(X<T, N>, int(*)[N], X<U, N>) {} // expected-note 1+{{candidate}}
 335   template<template<typename T, T> class X, typename T, typename U, T N>
 336     void x(int(*)[N], X<T, N>, X<U, N>) {} // expected-note 1+{{candidate}}
 337   int arr[3];
 338   void y() {
 339     x(X<int, 3>(), &arr, X<int, 3>());
 340     x(&arr, X<int, 3>(), X<int, 3>());
 341
 342     x(X<int, 3>(), &arr, X<char, 3>()); // expected-error {{no matching function}}
 343     x(&arr, X<int, 3>(), X<char, 3>()); // expected-error {{no matching function}}
 344
 345     x(X<char, 3>(), &arr, X<char, 3>());
 346     x(&arr, X<char, 3>(), X<char, 3>());
 347   }
 348 }
 349
 350 namespace nullptr_deduction {
 351   using nullptr_t = decltype(nullptr);
 352
 353   template<typename T, T v> struct X {};
 354   template<typename T, T v> void f(X<T, v>) {
 355     static_assert(!v, ""); // expected-warning 2{{implicit conversion of nullptr constant to 'bool'}}
 356   }
 357   void g() {
 358     f(X<int*, nullptr>()); // expected-note {{instantiation of}}
 359     f(X<nullptr_t, nullptr>()); // expected-note {{instantiation of}}
 360   }
 361
 362   template<template<typename T, T> class X, typename T, int *P>
 363     void f0(X<T, P>) {} // expected-note {{deduced non-type template argument does not have the same type as the corresponding template parameter ('std::nullptr_t' vs 'int *')}}
 364   void h0() {
 365     f0(X<int*, nullptr>());
 366     f0(X<nullptr_t, nullptr>()); // expected-error {{no matching function}}
 367   }
 368
 369   template<template<typename T, T> class X, typename T, typename U, int *P>
 370     void f1(X<T, P>, X<U, P>) {} // expected-note 2{{values of conflicting types}}
 371   void h() {
 372     f1(X<int*, nullptr>(), X<nullptr_t, nullptr>()); // expected-error {{no matching function}}
 373     f1(X<nullptr_t, nullptr>(), X<int*, nullptr>()); // expected-error {{no matching function}}
 374   }
 375
 376   template<template<typename T, T> class X, typename T, typename U, nullptr_t P>
 377     void f2(X<T, P>, X<U, P>) {} // expected-note 2{{values of conflicting types}}
 378   void i() {
 379     f2(X<int*, nullptr>(), X<nullptr_t, nullptr>()); // expected-error {{no matching function}}
 380     f2(X<nullptr_t, nullptr>(), X<int*, nullptr>()); // expected-error {{no matching function}}
 381   }
 382 }
 383
 384 namespace member_pointer {
 385   struct A { void f(int); };
 386   template<typename T, void (A::*F)(T)> struct B;
 387   template<typename T> struct C;
 388   template<typename T, void (A::*F)(T)> struct C<B<T, F>> {
 389     C() { A a; T t; (a.*F)(t); }
 390   };
 391   C<B<int, &A::f>> c;
 392 }
 393
 394 namespace deduction_substitution_failure {
 395   template<typename T> struct Fail { typedef typename T::error error; }; // expected-error 2{{prior to '::'}}
 396
 397   template<typename T, typename U> struct A {};
 398   template<typename T> struct A<T, typename Fail<T>::error> {}; // expected-note {{instantiation of}}
 399   A<int, int> ai; // expected-note {{during template argument deduction for class template partial specialization 'A<T, typename Fail<T>::error>' [with T = int]}} expected-note {{in instantiation of template class 'deduction_substitution_failure::A<int, int>'}}
 400
 401   template<typename T, typename U> int B; // expected-warning 0-1 {{extension}}
 402   template<typename T> int B<T, typename Fail<T>::error> {}; // expected-note {{instantiation of}}
 403   int bi = B<char, char>; // expected-note {{during template argument deduction for variable template partial specialization 'B<T, typename Fail<T>::error>' [with T = char]}}
 404 }
 405
 406 namespace deduction_after_explicit_pack {
 407   template<typename ...T, typename U> int *f(T ...t, int &r, U *u) {
 408     return u;
 409   }
 410   template<typename U, typename ...T> int *g(T ...t, int &r, U *u) {
 411     return u;
 412   }
 413   void h(float a, double b, int c) {
 414     f<float&, double&>(a, b, c, &c); // ok
 415     g<int, float&, double&>(a, b, c, &c); // ok
 416   }
 417
 418   template<class... ExtraArgs>
 419   int test(ExtraArgs..., unsigned vla_size, const char *input);
 420   int n = test(0, "");
 421
 422   template <typename... T> void i(T..., int, T..., ...); // expected-note 5{{deduced packs of different lengths}}
 423   void j() {
 424     i(0);
 425     i(0, 1); // expected-error {{no match}}
 426     i(0, 1, 2); // expected-error {{no match}}
 427     i<>(0);
 428     i<>(0, 1); // expected-error {{no match}}
 429     i<>(0, 1, 2); // expected-error {{no match}}
 430     i<int, int>(0, 1, 2, 3, 4);
 431     i<int, int>(0, 1, 2, 3, 4, 5); // expected-error {{no match}}
 432   }
 433
 434   // GCC alarmingly accepts this by deducing T={int} by matching the second
 435   // parameter against the first argument, then passing the first argument
 436   // through the first parameter.
 437   template<typename... T> struct X { X(int); operator int(); };
 438   template<typename... T> void p(T..., X<T...>, ...); // expected-note {{deduced packs of different lengths for parameter 'T' (<> vs. <int>)}}
 439   void q() { p(X<int>(0), 0); } // expected-error {{no match}}
 440
 441   struct A {
 442     template <typename T> void f(T, void *, int = 0); // expected-note 2{{no known conversion from 'double' to 'void *' for 2nd argument}}
 443     void f(); // expected-note 2{{requires 0}}
 444
 445     template <typename T> static void g(T, void *, int = 0); // expected-note 2{{no known conversion from 'double' to 'void *' for 2nd argument}}
 446     void g(); // expected-note 2{{requires 0}}
 447
 448     void h() {
 449       f(1.0, 2.0); // expected-error {{no match}}
 450       g(1.0, 2.0); // expected-error {{no match}}
 451     }
 452   };
 453   void f(A a) {
 454     a.f(1.0, 2.0); // expected-error {{no match}}
 455     a.g(1.0, 2.0); // expected-error {{no match}}
 456   }
 457 }
 458
 459 namespace overload_vs_pack {
 460   void f(int);
 461   void f(float);
 462   void g(double);
 463
 464   template<typename ...T> struct X {};
 465   template<typename ...T> void x(T...);
 466
 467   template<typename ...T> struct Y { typedef int type(typename T::error...); };
 468   template<> struct Y<int, float, double> { typedef int type; };
 469
 470   template<typename ...T> typename Y<T...>::type g1(X<T...>, void (*...fns)(T)); // expected-note {{deduced conflicting types for parameter 'T' (<int, float> vs. <(no value), double>)}}
 471   template<typename ...T> typename Y<T...>::type g2(void(*)(T...), void (*...fns)(T)); // expected-note {{deduced conflicting types for parameter 'T' (<int, float> vs. <(no value), double>)}}
 472
 473   template<typename T> int &h1(decltype(g1(X<int, float, T>(), f, f, g)) *p);
 474   template<typename T> float &h1(...);
 475
 476   template<typename T> int &h2(decltype(g2(x<int, float, T>, f, f, g)) *p);
 477   template<typename T> float &h2(...);
 478
 479   int n1 = g1(X<int, float>(), f, g); // expected-error {{no matching function}}
 480   int n2 = g2(x<int, float>, f, g); // expected-error {{no matching function}}
 481
 482   int &a1 = h1<double>(0); // ok, skip deduction for 'f's, deduce matching value from 'g'
 483   int &a2 = h2<double>(0);
 484
 485   float &b1 = h1<float>(0); // deduce mismatching value from 'g', so we do not trigger instantiation of Y
 486   float &b2 = h2<float>(0);
 487
 488   template<typename ...T> int partial_deduction(void (*...f)(T)); // expected-note {{deduced incomplete pack <(no value), double> for template parameter 'T'}}
 489   int pd1 = partial_deduction(f, g); // expected-error {{no matching function}}
 490
 491   template<typename ...T> int partial_deduction_2(void (*...f)(T), ...); // expected-note {{deduced incomplete pack <(no value), double> for template parameter 'T'}}
 492   int pd2 = partial_deduction_2(f, g); // expected-error {{no matching function}}
 493
 494   namespace cwg_example {
 495     void f(char, char);
 496     void f(int, int);
 497     void x(int, char);
 498
 499     template<typename T, typename ...U> void j(void(*)(U...), void (*...fns)(T, U));
 500     void test() { j(x, f, x); }
 501   }
 502 }
 503
 504 namespace b29946541 {
 505   template<typename> class A {};
 506   template<typename T, typename U, template<typename, typename> class C>
 507   void f(C<T, U>); // expected-note {{failed template argument deduction}}
 508   void g(A<int> a) { f(a); } // expected-error {{no match}}
 509 }
 510
 511 namespace deduction_from_empty_list {
 512   template<int M, int N = 5> void f(int (&&)[N], int (&&)[N]) { // expected-note {{1 vs. 2}}
 513     static_assert(M == N, "");
 514   }
 515
 516   void test() {
 517     f<5>({}, {});
 518     f<1>({}, {0});
 519     f<1>({0}, {});
 520     f<1>({0}, {0});
 521     f<1>({0}, {0, 1}); // expected-error {{no matching}}
 522   }
 523 }
 524
 525 namespace check_extended_pack {
 526   template<typename T> struct X { typedef int type; };
 527   template<typename ...T> void f(typename X<T>::type...);
 528   template<typename T> void f(T, int, int);
 529   void g() {
 530     f<int>(0, 0, 0);
 531   }
 532
 533   template<int, int*> struct Y {};
 534   template<int ...N> void g(Y<N...>); // expected-note {{deduced non-type template argument does not have the same type as the corresponding template parameter ('int *' vs 'int')}}
 535   int n;
 536   void h() { g<0>(Y<0, &n>()); } // expected-error {{no matching function}}
 537 }
 538
 539 namespace dependent_template_template_param_non_type_param_type {
 540   template<int N> struct A {
 541     template<typename V = int, V M = 12, V (*Y)[M], template<V (*v)[M]> class W>
 542     A(W<Y>);
 543   };
 544
 545   int n[12];
 546   template<int (*)[12]> struct Q {};
 547   Q<&n> qn;
 548   A<0> a(qn);
 549 }
 550
 551 namespace dependent_list_deduction {
 552   template<typename T, T V> void a(const int (&)[V]) {
 553     static_assert(is_same<T, decltype(sizeof(0))>::value, "");
 554     static_assert(V == 3, "");
 555   }
 556   template<typename T, T V> void b(const T (&)[V]) {
 557     static_assert(is_same<T, int>::value, "");
 558     static_assert(V == 3, "");
 559   }
 560   template<typename T, T V> void c(const T (&)[V]) {
 561     static_assert(is_same<T, decltype(sizeof(0))>::value, "");
 562     static_assert(V == 3, "");
 563   }
 564   void d() {
 565     a({1, 2, 3});
 566 #if __cplusplus <= 201402L
 567     // expected-error@-2 {{no match}} expected-note@-15 {{couldn't infer template argument 'T'}}
 568 #endif
 569     b({1, 2, 3});
 570     c({{}, {}, {}});
 571 #if __cplusplus <= 201402L
 572     // expected-error@-2 {{no match}} expected-note@-12 {{couldn't infer template argument 'T'}}
 573 #endif
 574   }
 575
 576   template<typename ...T> struct X;
 577   template<int ...T> struct Y;
 578   template<typename ...T, T ...V> void f(const T (&...p)[V]) {
 579     static_assert(is_same<X<T...>, X<int, char, char>>::value, "");
 580     static_assert(is_same<Y<V...>, Y<3, 2, 4>>::value, "");
 581   }
 582   template<typename ...T, T ...V> void g(const T (&...p)[V]) {
 583     static_assert(is_same<X<T...>, X<int, decltype(sizeof(0))>>::value, "");
 584     static_assert(is_same<Y<V...>, Y<2, 3>>::value, "");
 585   }
 586   void h() {
 587     f({1, 2, 3}, {'a', 'b'}, "foo");
 588     g({1, 2}, {{}, {}, {}});
 589 #if __cplusplus <= 201402
 590     // expected-error@-2 {{no match}}
 591     // expected-note@-9 {{deduced incomplete pack}}
 592     // We deduce V$1 = (size_t)3, which in C++1z also deduces T$1 = size_t.
 593 #endif
 594   }
 595 }
 596
 597 namespace designators {
 598   template<typename T, int N> constexpr int f(T (&&)[N]) { return N; } // expected-note 2{{couldn't infer template argument 'T'}}
 599   static_assert(f({1, 2, [20] = 3}) == 3, ""); // expected-error {{no matching function}} expected-warning 2{{C99}} expected-note {{}}
 600
 601   static_assert(f({.a = 1, .b = 2}) == 3, ""); // expected-error {{no matching function}}
 602 }
 603
 604 namespace nested_packs {
 605   template<typename ...T, typename ...U> void f(T (*...f)(U...)); // expected-note {{deduced packs of different lengths for parameter 'U' (<> vs. <int>)}}
 606   void g() { f(g); f(g, g); f(g, g, g); }
 607   void h(int) { f(h); f(h, h); f(h, h, h); }
 608   void i() { f(g, h); } // expected-error {{no matching function}}
 609
 610 #if __cplusplus >= 201703L
 611   template<auto ...A> struct Q {};
 612   template<typename ...T, T ...A, T ...B> void q(Q<A...>, Q<B...>); // #q
 613   void qt(Q<> q0, Q<1, 2> qii, Q<1, 2, 3> qiii) {
 614     q(q0, q0);
 615     q(qii, qii);
 616     q(qii, qiii); // expected-error {{no match}} expected-note@#q {{deduced packs of different lengths for parameter 'T' (<int, int> vs. <int, int, int>)}}
 617     q(q0, qiii); // expected-error {{no match}} expected-note@#q {{deduced packs of different lengths for parameter 'T' (<> vs. <int, int, int>)}}
 618   }
 619 #endif
 620 }
 621
 622 namespace PR44890 {
 623   template<typename ...Ts>
 624     struct tuple {};
 625
 626   template<int I, typename ...Ts>
 627     int get0(const tuple<Ts...> &t) { return 0; }
 628
 629   template<typename ...Ts> struct tuple_wrapper : tuple<Ts...> {
 630     template<int I> int get() { return get0<0, Ts...>(*this); }
 631   };
 632
 633   int f() {
 634     tuple_wrapper<int> w;
 635     return w.get<0>();
 636   }
 637 }
 638
 639 namespace merge_size_only_deductions {
 640 #if __cplusplus >= 201703L
 641   // Based on a testcase by Hubert Tong.
 642   template<typename ...> struct X {};
 643   template<auto ...> struct Y {};
 644   template<typename T> struct id { using Type = T; };
 645
 646   template<typename ...T, typename T::Type ...V>
 647     int f(X<char [V] ...>, Y<V ...>, X<T ...>);
 648
 649   using size_t = __SIZE_TYPE__;
 650   int a = f(X<char [1], char [2]>(), Y<(size_t)1, (size_t)2>(), X<id<size_t>, id<size_t>>());
 651   int b = f(X<char [1], char [2]>(), Y<1, 2>(), X<id<int>, id<int>>());
 652 #endif
 653 }
 654
 655 namespace PR49724 {
 656   struct A;
 657   template<int A::*> class X {};
 658   template<int A::*P> void f(X<P>);
 659   void g(X<nullptr> x) { f(x); }
 660
 661   template<void (A::*)()> class Y {};
 662   template<void (A::*P)()> void f(Y<P>);
 663   void g(Y<nullptr> y) { f(y); }
 664 }
 665
 666 namespace sugared_deduction {
 667 using Int = int;
 668
 669 template <class T, int C> void f1(T(&)[C], T(&)[C+1]);
 670 // expected-note@-1 {{candidate template ignored: deduced type 'int[3]' of 2nd parameter does not match adjusted type 'Int[2]' (aka 'int[2]') of argument [with T = Int, C = 2]}}
 671
 672 void t1() {
 673   Int a[2], b[2];
 674   f1(a, b); // expected-error {{no matching function for call to 'f1'}}
 675 }
 676
 677 #if defined(__cpp_concepts)
 678 template <class T> void f2() requires false {}
 679 // expected-note@-1 {{candidate template ignored: constraints not satisfied [with T = Int]}}
 680 // expected-note@-2 {{because 'false' evaluated to false}}
 681
 682 void t2() {
 683   f2<Int>(); // expected-error {{no matching function for call to 'f2'}}
 684 }
 685 #endif
 686 } // namespace sugared_deduction