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

Fixes #78694

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -666,6 +666,8 @@ Bug Fixes to C++ Support
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
 - Fixed expression transformation for ``[[assume(...)]]``, allowing using pack indexing expressions within the
   assumption if they also occur inside of a dependent lambda. (#GH114787)
+- Clang now uses valid deduced type locations when diagnosing functions with trailing return type
+  missing placeholder return type. (#GH78694)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaType.cpp

@@ -4882,9 +4882,17 @@ 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();
+            // Attach a valid source location for diagnostics on functions with
+            // trailing return types missing 'auto'. Attempt to get the location
+            // from the declared type; if invalid, fall back to the trailing
+            // return type's location.
+            SourceLocation Loc = D.getDeclSpec().getTypeSpecTypeLoc();
+            SourceRange SR = D.getDeclSpec().getSourceRange();
+            if (Loc.isInvalid()) {
+              Loc = FTI.getTrailingReturnTypeLoc();
+              SR = D.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;

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'}}
+};
+}