[Clang][Sema] Fix a bug on template partial specialization with issue on deduction of nontype template parameter

[jcsxky]

Fix #68885
When build expression from a deduced argument whose kind is Declaration and NTTPType(which declared as decltype(auto)) is deduced as a reference type, BuildExpressionFromDeclTemplateArgument just create a DeclRef. This is incorrect while we get type from the expression since we can't get the original reference type from DeclRef. Creating a SubstNonTypeTemplateParmExpr expression and make the deduction correct. Replacement expression of SubstNonTypeTemplateParmExpr just helps the deduction and may not be same with the original expression.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Qizhi Hu (jcsxky)

Changes

Fix #68885
When build expression from a deduced argument whose kind is Declaration and NTTPType(which declared as decltype(auto)) is deduced as a reference type, BuildExpressionFromDeclTemplateArgument just create a DeclRef. This is incorrect while we get type from the expression since we can't get the original reference type from DeclRef. Creating a ParenExpr expression and make the deduction correct. ParenExpr expression just helps the deduction and may not be same with the original expression.

---

Full diff: https://github.com/llvm/llvm-project/pull/90376.diff

5 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/include/clang/Sema/Sema.h (+6-4)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+13-4)
• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+6-4)
• (added) clang/test/SemaCXX/PR68885.cpp (+21)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a1390d6536b28c..8c1ba2a50b921b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -588,6 +588,8 @@ Bug Fixes to C++ Support
 - Fixed a use-after-free bug in parsing of type constraints with default arguments that involve lambdas. (#GH67235)
 - Fixed bug in which the body of a consteval lambda within a template was not parsed as within an
   immediate function context.
+- Fix a bug on template partial specialization with issue on deduction of nontype tempalte parameter
+  whose type is `decltype(auto)`. Fixes (#GH68885).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 1ca523ec88c2f9..809b9c4498f697 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -9249,7 +9249,8 @@ class Sema final : public SemaBase {
   void NoteTemplateParameterLocation(const NamedDecl &Decl);
 
   ExprResult BuildExpressionFromDeclTemplateArgument(
-      const TemplateArgument &Arg, QualType ParamType, SourceLocation Loc);
+      const TemplateArgument &Arg, QualType ParamType, SourceLocation Loc,
+      NamedDecl *TemplateParam = nullptr);
   ExprResult
   BuildExpressionFromNonTypeTemplateArgument(const TemplateArgument &Arg,
                                              SourceLocation Loc);
@@ -9572,9 +9573,10 @@ class Sema final : public SemaBase {
 
   bool isSameOrCompatibleFunctionType(QualType Param, QualType Arg);
 
-  TemplateArgumentLoc getTrivialTemplateArgumentLoc(const TemplateArgument &Arg,
-                                                    QualType NTTPType,
-                                                    SourceLocation Loc);
+  TemplateArgumentLoc
+  getTrivialTemplateArgumentLoc(const TemplateArgument &Arg, QualType NTTPType,
+                                SourceLocation Loc,
+                                NamedDecl *TemplateParam = nullptr);
 
   /// Get a template argument mapping the given template parameter to itself,
   /// e.g. for X in \c template<int X>, this would return an expression template
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index bbcb7c33a98579..6a2f025521ce8e 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -8438,10 +8438,9 @@ void Sema::NoteTemplateParameterLocation(const NamedDecl &Decl) {
 /// declaration and the type of its corresponding non-type template
 /// parameter, produce an expression that properly refers to that
 /// declaration.
-ExprResult
-Sema::BuildExpressionFromDeclTemplateArgument(const TemplateArgument &Arg,
-                                              QualType ParamType,
-                                              SourceLocation Loc) {
+ExprResult Sema::BuildExpressionFromDeclTemplateArgument(
+    const TemplateArgument &Arg, QualType ParamType, SourceLocation Loc,
+    NamedDecl *TemplateParam) {
   // C++ [temp.param]p8:
   //
   //   A non-type template-parameter of type "array of T" or
@@ -8508,6 +8507,16 @@ Sema::BuildExpressionFromDeclTemplateArgument(const TemplateArgument &Arg,
   } else {
     assert(ParamType->isReferenceType() &&
            "unexpected type for decl template argument");
+    if (ParamType->isLValueReferenceType())
+      if (NonTypeTemplateParmDecl *NTTP =
+              dyn_cast_if_present<NonTypeTemplateParmDecl>(TemplateParam)) {
+        QualType TemplateParamType = NTTP->getType();
+        const AutoType *AT = TemplateParamType->getAs<AutoType>();
+        if (AT && AT->isDecltypeAuto())
+          RefExpr = new (getASTContext())
+              ParenExpr(RefExpr.get()->getBeginLoc(),
+                        RefExpr.get()->getEndLoc(), RefExpr.get());
+      }
   }
 
   // At this point we should have the right value category.
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index c3815bca038554..e93f7bd842e444 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2639,7 +2639,8 @@ static bool isSameTemplateArg(ASTContext &Context,
 /// argument.
 TemplateArgumentLoc
 Sema::getTrivialTemplateArgumentLoc(const TemplateArgument &Arg,
-                                    QualType NTTPType, SourceLocation Loc) {
+                                    QualType NTTPType, SourceLocation Loc,
+                                    NamedDecl *TemplateParam) {
   switch (Arg.getKind()) {
   case TemplateArgument::Null:
     llvm_unreachable("Can't get a NULL template argument here");
@@ -2651,7 +2652,8 @@ Sema::getTrivialTemplateArgumentLoc(const TemplateArgument &Arg,
   case TemplateArgument::Declaration: {
     if (NTTPType.isNull())
       NTTPType = Arg.getParamTypeForDecl();
-    Expr *E = BuildExpressionFromDeclTemplateArgument(Arg, NTTPType, Loc)
+    Expr *E = BuildExpressionFromDeclTemplateArgument(Arg, NTTPType, Loc,
+                                                      TemplateParam)
                   .getAs<Expr>();
     return TemplateArgumentLoc(TemplateArgument(E), E);
   }
@@ -2718,8 +2720,8 @@ static bool ConvertDeducedTemplateArgument(
     // Convert the deduced template argument into a template
     // argument that we can check, almost as if the user had written
     // the template argument explicitly.
-    TemplateArgumentLoc ArgLoc =
-        S.getTrivialTemplateArgumentLoc(Arg, QualType(), Info.getLocation());
+    TemplateArgumentLoc ArgLoc = S.getTrivialTemplateArgumentLoc(
+        Arg, QualType(), Info.getLocation(), Param);
 
     // Check the template argument, converting it as necessary.
     return S.CheckTemplateArgument(
diff --git a/clang/test/SemaCXX/PR68885.cpp b/clang/test/SemaCXX/PR68885.cpp
new file mode 100644
index 00000000000000..4ff95771e2caf5
--- /dev/null
+++ b/clang/test/SemaCXX/PR68885.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+
+// expected-no-diagnostics
+
+template <decltype(auto) a>
+struct S {
+    static constexpr int i = 42;
+};
+
+template <decltype(auto) a> requires true
+struct S<a> {
+    static constexpr int i = 0;
+};
+
+static constexpr int a = 0;
+
+void test() {
+    static_assert(S<a>::i == 0);
+    static_assert(S<(a)>::i == 0);
+    static_assert(S<((a))>::i == 0);
+}

[Endilll]

Sema.h changes look good to me.

[zyn0217]

Did you try to preserve the references using SubstNonTypeTemplateParmExprs just like what @cor3ntin said? This looks a bit odd to me...

[jcsxky]

Ah, thanks for your remind! SubstNonTypeTemplateParmExpr is more suitable here.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.