[clang][Sema] Fix crash when diagnosing candidates with parameter packs

clang/docs/ReleaseNotes.rst

@@ -734,7 +734,6 @@ Bug Fixes to C++ Support
   from being explicitly specialized for a given implicit instantiation of the class template.
 - Fixed a crash when ``this`` is used in a dependent class scope function template specialization
   that instantiates to a static member function.
-
 - Fix crash when inheriting from a cv-qualified type. Fixes #GH35603
 - Fix a crash when the using enum declaration uses an anonymous enumeration. Fixes (#GH86790).
 - Handled an edge case in ``getFullyPackExpandedSize`` so that we now avoid a false-positive diagnostic. (#GH84220)
@@ -796,6 +795,8 @@ Bug Fixes to C++ Support
   Fixes (#GH91308).
 - Fix a crash caused by a regression in the handling of ``source_location``
   in dependent contexts. Fixes (#GH92680).
+- Fixed a crash when diagnosing failed conversions involving template parameter
+  packs. (#GH93076)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaOverload.cpp

@@ -13,6 +13,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DependenceFlags.h"
@@ -11301,8 +11302,16 @@ static void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand,
   Expr *FromExpr = Conv.Bad.FromExpr;
   QualType FromTy = Conv.Bad.getFromType();
   QualType ToTy = Conv.Bad.getToType();
-  SourceRange ToParamRange =
-      !isObjectArgument ? Fn->getParamDecl(I)->getSourceRange() : SourceRange();
+  SourceRange ToParamRange;
+
+  // FIXME: In presence of parameter packs we can't determine parameter range
+  // reliably, as we don't have access to instantiation.
+  bool HasParamPack =
+      llvm::any_of(Fn->parameters().take_front(I), [](const ParmVarDecl *Parm) {
+        return Parm->isParameterPack();
+      });
+  if (!isObjectArgument && !HasParamPack)
+    ToParamRange = Fn->getParamDecl(I)->getSourceRange();
 
   if (FromTy == S.Context.OverloadTy) {
     assert(FromExpr && "overload set argument came from implicit argument?");

clang/test/SemaCXX/overload-template.cpp

@@ -58,3 +58,13 @@ namespace overloadCheck{
   }
 }
 #endif
+
+namespace GH93076 {
+template <typename ...a> int b(a..., int); // expected-note-re 3 {{candidate function template not viable: no known conversion from 'int ()' to 'int' for {{.*}} argument}}
+int d() {
+  (void)b<int, int>(0, 0, d); // expected-error {{no matching function for call to 'b'}}
+  (void)b<int, int>(0, d, 0); // expected-error {{no matching function for call to 'b'}}
+  (void)b<int, int>(d, 0, 0); // expected-error {{no matching function for call to 'b'}}
+  return 0;
+ }
+}