[clang] [Clang] Fix crash on non-parameter NTTP arguments during partial ordering (PR #231599)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Emmanuel Antonio (EmmanuelAC1)

<details>
<summary>Changes</summary>

0b128824e723 (#<!-- -->224291) added a CWG2900 check in Sema::CheckTemplateArgument that calls isUndeducedAuto(getTypeOfConstantTemplateParameter(Arg), true). When Arg does not name a constant template parameter (for example, when checking a partial specialization argument such as 0, an enum value, nullptr, or a variable against an auto NTTP), getTypeOfConstantTemplateParameter returns a null QualType, which isUndeducedAuto immediately dereferenced.

Guard against a null QualType in isUndeducedAuto and check that Arg is an Expression before calling Arg.getAsExpr() in getTypeOfConstantTemplateParameter.

---
Full diff: https://github.com/llvm/llvm-project/pull/231599.diff


3 Files Affected:

- (modified) clang/lib/Sema/SemaTemplate.cpp (+2) 
- (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+4-3) 
- (modified) clang/test/SemaTemplate/temp_arg_nontype_arg_type.cpp (+16) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index ec1f8077893eb..5a61cb852d15e 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -5613,6 +5613,8 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param, TemplateArgumentLoc &ArgLoc,
     //   parameter of type decltype(auto) ([temp.func.order]), the declaration
     //   is ill-formed.
     auto isUndeducedAuto = [](QualType T, bool DecltypeAuto) {
+      if (T.isNull())
+        return false;
       const AutoType *AT = T->getContainedAutoType();
       return AT && AT->isDecltypeAuto() == DecltypeAuto &&
              AT->getDeducedType().isNull();
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index f18097e358ca4..90724e1d01cb2 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -257,9 +257,10 @@ getNTTParameterFromExpr(const Expr *E, UnsignedOrNone Depth) {
 }
 
 QualType Sema::getTypeOfConstantTemplateParameter(const TemplateArgument &Arg) {
-  if (NonTypeOrVarTemplateParmDecl NTTP =
-          getNTTParameterFromExpr(Arg.getAsExpr(), std::nullopt))
-    return NTTP.getType();
+  if (Arg.getKind() == TemplateArgument::Expression)
+    if (NonTypeOrVarTemplateParmDecl NTTP =
+            getNTTParameterFromExpr(Arg.getAsExpr(), std::nullopt))
+      return NTTP.getType();
   return QualType();
 }
 
diff --git a/clang/test/SemaTemplate/temp_arg_nontype_arg_type.cpp b/clang/test/SemaTemplate/temp_arg_nontype_arg_type.cpp
index 01290d91a2e84..e1d16a1d7689e 100644
--- a/clang/test/SemaTemplate/temp_arg_nontype_arg_type.cpp
+++ b/clang/test/SemaTemplate/temp_arg_nontype_arg_type.cpp
@@ -60,4 +60,20 @@ namespace ex7 {
   short *g1(G<(j)> *ap) { return f(ap); }
   long *g2(G<j> *ap) { return f(ap); }
 }
+
+namespace partial_spec {
+  enum E { Val };
+  int global_var;
+
+  template <class T, auto V> struct A {};
+  template <class T> struct A<T, 0> {};
+  template <class T> struct A<T, Val> {};
+  template <class T> struct A<T, nullptr> {};
+
+  template <class T, auto... V> struct Pack {};
+  template <class T> struct Pack<T, 0, 1> {};
+
+  template <class T, const auto &V> struct Ref {};
+  template <class T> struct Ref<T, global_var> {};
+}
 #endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/231599