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

Prevent OOB access by not printing target parameter range when there's a
pack in the function parameters.

Fixes #93076.
Fixes #76354.
Fixes #70191.

Author: kadircet

clang/docs/ReleaseNotes.rst

@@ -730,7 +730,6 @@ 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)
@@ -790,6 +789,8 @@ Bug Fixes to C++ Support
   Fixes (#GH87210), (GH89541).
 - Clang no longer tries to check if an expression is immediate-escalating in an unevaluated context.
   Fixes (#GH91308).
+- 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 +11302,14 @@ 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 +58,6 @@ namespace overloadCheck{
   }
 }
 #endif
+
+template <typename ...a> int b(a...); // expected-note {{candidate function template not viable: no known conversion from 'int ()' to 'int' for 2nd argument}}
+int d() { return b<int, int>(0, d); } // expected-error {{no matching function for call to 'b'}}