[Clang] Clarify diagnostic notes for implicitly generated deduction guides (llvm#96084)

Given the following invalid code,
```cpp
template <class T>
struct S {
  T *a;
};
S s = {1};
```
we produce such diagnostics currently:
```
<source>:2:8: note: candidate template ignored: could not match 'S<T>' against 'int'
    2 | struct S {
      |        ^
<source>:2:8: note: candidate template ignored: could not match 'T *' against 'int'
```
Which I think is confusing because there's no `S<T>` nor `T *` at the
location it points to. This is because we're deducing the initializer
against implicitly generated deduction guides, and their source
locations just point to the corresponding `RecordDecl`. Hence the
misleading notes.

This patch alleviates the issue by adding extra notes demonstrating
which implicit deduction guide we're deducing against. In other words,
in addition to the note of `could not match 'T *' against 'int'`, we
would also say the implicit deduction guide we're trying to use:
`template <class T> S(T *) -> S<T>`, which looks clearer IMO.

---------

Co-authored-by: Sirraide <aeternalmail@gmail.com>

Author: 2 people

clang/docs/ReleaseNotes.rst

@@ -651,6 +651,8 @@ Improvements to Clang's diagnostics
   that will be destroyed at the end of the full expression.
   Fixes #GH54492.
 
+- Clang now shows implicit deduction guides when diagnosing overload resolution failure. #GH92393.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2419,6 +2419,7 @@ def err_selected_explicit_constructor : Error<
   "chosen constructor is explicit in copy-initialization">;
 def note_explicit_ctor_deduction_guide_here : Note<
   "explicit %select{constructor|deduction guide}0 declared here">;
+def note_implicit_deduction_guide : Note<"implicit deduction guide declared as '%0'">;
 
 // C++11 auto
 def warn_cxx98_compat_auto_type_specifier : Warning<

clang/lib/Sema/SemaOverload.cpp

@@ -40,6 +40,7 @@
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/STLForwardCompat.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
@@ -11884,6 +11885,46 @@ static void DiagnoseFailedExplicitSpec(Sema &S, OverloadCandidate *Cand) {
       << (ES.getExpr() ? ES.getExpr()->getSourceRange() : SourceRange());
 }
 
+static void NoteImplicitDeductionGuide(Sema &S, FunctionDecl *Fn) {
+  auto *DG = dyn_cast<CXXDeductionGuideDecl>(Fn);
+  if (!DG)
+    return;
+  TemplateDecl *OriginTemplate =
+      DG->getDeclName().getCXXDeductionGuideTemplate();
+  // We want to always print synthesized deduction guides for type aliases.
+  // They would retain the explicit bit of the corresponding constructor.
+  if (!(DG->isImplicit() || (OriginTemplate && OriginTemplate->isTypeAlias())))
+    return;
+  std::string FunctionProto;
+  llvm::raw_string_ostream OS(FunctionProto);
+  FunctionTemplateDecl *Template = DG->getDescribedFunctionTemplate();
+  if (!Template) {
+    // This also could be an instantiation. Find out the primary template.
+    FunctionDecl *Pattern =
+        DG->getTemplateInstantiationPattern(/*ForDefinition=*/false);
+    if (!Pattern) {
+      // The implicit deduction guide is built on an explicit non-template
+      // deduction guide. Currently, this might be the case only for type
+      // aliases.
+      // FIXME: Add a test once https://github.com/llvm/llvm-project/pull/96686
+      // gets merged.
+      assert(OriginTemplate->isTypeAlias() &&
+             "Non-template implicit deduction guides are only possible for "
+             "type aliases");
+      DG->print(OS);
+      S.Diag(DG->getLocation(), diag::note_implicit_deduction_guide)
+          << FunctionProto;
+      return;
+    }
+    Template = Pattern->getDescribedFunctionTemplate();
+    assert(Template && "Cannot find the associated function template of "
+                       "CXXDeductionGuideDecl?");
+  }
+  Template->print(OS);
+  S.Diag(DG->getLocation(), diag::note_implicit_deduction_guide)
+      << FunctionProto;
+}
+
 /// Generates a 'note' diagnostic for an overload candidate.  We've
 /// already generated a primary error at the call site.
 ///
@@ -11941,6 +11982,17 @@ static void NoteFunctionCandidate(Sema &S, OverloadCandidate *Cand,
     return;
   }
 
+  // If this is a synthesized deduction guide we're deducing against, add a note
+  // for it. These deduction guides are not explicitly spelled in the source
+  // code, so simply printing a deduction failure note mentioning synthesized
+  // template parameters or pointing to the header of the surrounding RecordDecl
+  // would be confusing.
+  //
+  // We prefer adding such notes at the end of the deduction failure because
+  // duplicate code snippets appearing in the diagnostic would likely become
+  // noisy.
+  auto _ = llvm::make_scope_exit([&] { NoteImplicitDeductionGuide(S, Fn); });
+
   switch (Cand->FailureKind) {
   case ovl_fail_too_many_arguments:
   case ovl_fail_too_few_arguments:

clang/test/CXX/drs/cwg26xx.cpp

@@ -193,8 +193,11 @@ static_assert(__is_same(decltype(i), I<char, 4>));
 J j = { "ghi" };
 // since-cxx20-error@-1 {{no viable constructor or deduction guide}}
 //   since-cxx20-note@#cwg2681-J {{candidate template ignored: could not match 'J<N>' against 'const char *'}}
+//   since-cxx20-note@#cwg2681-J {{implicit deduction guide declared as 'template <size_t N> J(J<N>) -> J<N>'}}
 //   since-cxx20-note@#cwg2681-J {{candidate template ignored: could not match 'const unsigned char' against 'const char'}}
+//   since-cxx20-note@#cwg2681-J {{implicit deduction guide declared as 'template <size_t N> J(const unsigned char (&)[N]) -> J<N>'}}
 //   since-cxx20-note@#cwg2681-J {{candidate function template not viable: requires 0 arguments, but 1 was provided}}
+//   since-cxx20-note@#cwg2681-J {{implicit deduction guide declared as 'template <size_t N> J() -> J<N>'}}
 #endif
 }
 

clang/test/CXX/expr/expr.post/expr.type.conv/p1.cpp

@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -std=c++1z -verify %s
 
-template<typename T> struct A { // expected-note 2{{candidate}}
+template<typename T> struct A { // expected-note 2{{candidate}} expected-note 2{{implicit deduction guide}}
   T t, u;
 };
 template<typename T> A(T, T) -> A<T>; // expected-note {{deduced conflicting types for parameter 'T'}}

clang/test/CXX/over/over.match/over.match.funcs/over.match.class.deduct/p2.cpp

@@ -35,8 +35,9 @@ namespace std {
 }
 
 namespace p0702r1 {
-  template<typename T> struct X { // expected-note {{candidate}}
-    X(std::initializer_list<T>); // expected-note {{candidate template ignored: could not match 'std::initializer_list<T>' against 'Z'}}
+  template<typename T> struct X { // expected-note {{candidate}} expected-note {{implicit deduction guide}}
+    X(std::initializer_list<T>); // expected-note {{candidate template ignored: could not match 'std::initializer_list<T>' against 'Z'}} \
+                                 // expected-note {{implicit deduction guide declared as 'template <typename T> X(std::initializer_list<T>) -> X<T>'}}
   };
 
   X xi = {0};

clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.call/p3-0x.cpp

@@ -6,8 +6,8 @@
 #if __cplusplus > 201402L
 namespace ClassTemplateParamNotForwardingRef {
   // This is not a forwarding reference.
-  template<typename T> struct A { // expected-note {{candidate}}
-    A(T&&); // expected-note {{expects an rvalue}}
+  template<typename T> struct A { // expected-note {{candidate}} expected-note {{implicit deduction guide}}
+    A(T&&); // expected-note {{expects an rvalue}} expected-note {{implicit deduction guide}}
   };
   int n;
   A a = n; // expected-error {{no viable constructor or deduction guide}}
@@ -75,10 +75,12 @@ namespace std_example {
   int n3 = g(i); // expected-error{{no matching function for call to 'g'}}
 
 #if __cplusplus > 201402L
-  template<class T> struct A { // expected-note {{candidate}}
+  template<class T> struct A { // expected-note {{candidate}} expected-note {{implicit deduction guide}}
     template<class U>
-    A(T &&, U &&, int *); // expected-note {{[with T = int, U = int] not viable: expects an rvalue}}
-    A(T &&, int *);       // expected-note {{requires 2}}
+    A(T &&, U &&, int *); // expected-note {{[with T = int, U = int] not viable: expects an rvalue}} \
+                          // expected-note {{implicit deduction guide declared as 'template <class T, class U> A(T &&, type-parameter-0-1 &&, int *) -> A<T>'}}
+    A(T &&, int *);       // expected-note {{requires 2}} \
+                          // expected-note {{implicit deduction guide declared as 'template <class T> A(T &&, int *) -> A<T>'}}
   };
   template<class T> A(T &&, int *) -> A<T>; // expected-note {{requires 2}}
 

clang/test/Modules/template_name_lookup.cpp

@@ -7,5 +7,7 @@ import foo;
 void use() {
   X x; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'X'}}
        // expected-note@Inputs/template_name_lookup/foo.cppm:3 {{candidate template ignored: couldn't infer template argument 'T'}}
+       // expected-note@Inputs/template_name_lookup/foo.cppm:3 {{implicit deduction guide declared as 'template <typename T> X(X<T>) -> X<T>'}}
        // expected-note@Inputs/template_name_lookup/foo.cppm:3 {{candidate function template not viable: requires 1 argument, but 0 were provided}}
+       // expected-note@Inputs/template_name_lookup/foo.cppm:3 {{implicit deduction guide declared as 'template <typename T> X() -> X<T>'}}
 }

clang/test/PCH/cxx-explicit-specifier.cpp

@@ -79,8 +79,8 @@ struct A {
 B<true> b_true;
 B<false> b_false;
 #else
-//expected-note@-8 {{candidate template ignored}}
-//expected-note@-8 {{explicit constructor declared here}}
+//expected-note@-8 {{candidate template ignored}} expected-note@-8 {{implicit deduction guide declared as 'template <bool b> A(A<b>) -> A<b>'}}
+//expected-note@-8 {{explicit constructor declared here}} expected-note@-8 {{implicit deduction guide declared as 'template <bool b> explicit(b) A(B<b>) -> A<b>'}}
 //expected-note@-15+ {{candidate constructor}}
 //expected-note@-8+ {{explicit conversion function is not a candidate (explicit specifier}}
 //expected-note@-11 {{explicit constructor is not a candidate (explicit specifier}}

clang/test/Sema/tls_alignment.cpp

@@ -22,7 +22,9 @@ struct  struct_with_aligned_field {
 template <typename>
 struct templated_struct {};
 // expected-note@-1{{candidate template ignored: couldn't infer template argument ''}}
-// expected-note@-2{{candidate function template not viable: requires 1 argument, but 0 were provided}}
+// expected-note@-2{{implicit deduction guide declared as 'template <typename> templated_struct() -> templated_struct<type-parameter-0-0>'}}
+// expected-note@-3{{candidate function template not viable: requires 1 argument, but 0 were provided}}
+// expected-note@-4{{implicit deduction guide declared as 'template <typename> templated_struct(templated_struct<type-parameter-0-0>) -> templated_struct<type-parameter-0-0>'}}
 
 // A typedef of the aligned struct.
 typedef aligned_struct another_aligned_struct;