[clang] [Clang] enhance diagnostic by attaching source location to deduced type in trailing return without auto (PR #115786)

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk created https://github.com/llvm/llvm-project/pull/115786

Fixes #78694

>From da1b3982e84114cb1214ca5c3d8ed520d1589b83 Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Tue, 12 Nov 2024 00:59:37 +0200
Subject: [PATCH] [Clang] enhance diagnostic by attaching source location to
 deduced type in trailing return without auto

---
 clang/docs/ReleaseNotes.rst                       |  1 +
 clang/lib/Sema/SemaType.cpp                       | 15 ++++++++++++---
 .../CXX/dcl.decl/dcl.meaning/dcl.fct/p2-cxx0x.cpp | 13 ++++++++++++-
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index e235a04f78112b..f4cd7f99cdce14 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -652,6 +652,7 @@ Bug Fixes to C++ Support
   an implicitly instantiated class template specialization. (#GH51051)
 - Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
+- Clang now uses deduced type locations in trailing return without auto diagnostics. (#GH78694)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 515b9f689a248a..c50d15a3adc442 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -4887,9 +4887,18 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
                       cast<AutoType>(T)->getKeyword() !=
                           AutoTypeKeyword::Auto ||
                       cast<AutoType>(T)->isConstrained())) {
-            S.Diag(D.getDeclSpec().getTypeSpecTypeLoc(),
-                   diag::err_trailing_return_without_auto)
-                << T << D.getDeclSpec().getSourceRange();
+            SourceLocation Loc = D.getDeclSpec().getTypeSpecTypeLoc();
+            SourceRange SR = D.getDeclSpec().getSourceRange();
+            if (Loc.isInvalid()) {
+              TypeSourceInfo *TSI = nullptr;
+              S.GetTypeFromParser(FTI.getTrailingReturnType(), &TSI);
+              if (TSI) {
+                TypeLoc TSILoc = TSI->getTypeLoc();
+                Loc = TSILoc.getBeginLoc();
+                SR = TSILoc.getSourceRange();
+              }
+            }
+            S.Diag(Loc, diag::err_trailing_return_without_auto) << T << SR;
             D.setInvalidType(true);
             // FIXME: recover and fill decls in `TypeLoc`s.
             AreDeclaratorChunksValid = false;
diff --git a/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p2-cxx0x.cpp b/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p2-cxx0x.cpp
index ce90728861605f..e43eb8e48c7272 100644
--- a/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p2-cxx0x.cpp
+++ b/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p2-cxx0x.cpp
@@ -1,7 +1,18 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s 
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify %s
+// RUN: not %clang_cc1 -fsyntax-only -std=c++11 -fno-diagnostics-show-line-numbers -fcaret-diagnostics-max-lines=1 %s 2>&1 | FileCheck %s -strict-whitespace
 
 auto a() -> int; // ok
 const auto b() -> int; // expected-error {{function with trailing return type must specify return type 'auto', not 'const auto'}}
 auto *c() -> int; // expected-error {{function with trailing return type must specify return type 'auto', not 'auto *'}}
 auto (d() -> int); // expected-error {{trailing return type may not be nested within parentheses}}
 auto e() -> auto (*)() -> auto (*)() -> void; // ok: same as void (*(*e())())();
+
+namespace GH78694 {
+
+template <typename T> struct B {
+  // CHECK:      error: function with trailing return type must specify return type 'auto', not 'void'
+  // CHECK-NEXT: {{^}}  template <class U> B(U) -> B<int>;
+  // CHECK-NEXT: {{^}}                             ^~~~~~{{$}}
+  template <class U> B(U) -> B<int>; // expected-error {{function with trailing return type must specify return type 'auto', not 'void'}}
+};
+}