[Clang][Sema] Fix a bug on template partial specialization with issue on deduction of nontype template parameter (#90376)

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.

Co-authored-by: huqizhi <[email protected]>

Author: jcsxky

clang/docs/ReleaseNotes.rst

@@ -612,6 +612,8 @@ Bug Fixes to C++ Support
 - Fix CTAD for ``std::initializer_list``. This allows ``std::initializer_list{1, 2, 3}`` to be deduced as
   ``std::initializer_list<int>`` as intended.
 - Fix a bug on template partial specialization whose template parameter is `decltype(auto)`.
+- Fix a bug on template partial specialization with issue on deduction of nontype template parameter
+  whose type is `decltype(auto)`. Fixes (#GH68885).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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

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 +8507,18 @@ Sema::BuildExpressionFromDeclTemplateArgument(const TemplateArgument &Arg,
   } else {
     assert(ParamType->isReferenceType() &&
            "unexpected type for decl template argument");
+    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()) SubstNonTypeTemplateParmExpr(
+            ParamType->getPointeeType(), RefExpr.get()->getValueKind(),
+            RefExpr.get()->getExprLoc(), RefExpr.get(), VD, NTTP->getIndex(),
+            /*PackIndex=*/std::nullopt,
+            /*RefParam=*/true);
+      }
+    }
   }
 
   // At this point we should have the right value category.

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(

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);
+}