[clang] Represent array refs as TemplateArgument::Declaration (llvm#80050)

This returns (probably temporarily) array-referring NTTP behavior to
which was prior to llvm#78041 because ~~I'm fed up~~ have no time to fix
regressions.

(cherry picked from commit 9bf4e54)

Author: bolshakov-a

clang/lib/Sema/SemaTemplate.cpp

@@ -7412,9 +7412,9 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     if (ArgResult.isInvalid())
       return ExprError();
 
-    // Prior to C++20, enforce restrictions on possible template argument
-    // values.
-    if (!getLangOpts().CPlusPlus20 && Value.isLValue()) {
+    if (Value.isLValue()) {
+      APValue::LValueBase Base = Value.getLValueBase();
+      auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
       //   For a non-type template-parameter of pointer or reference type,
       //   the value of the constant expression shall not refer to
       assert(ParamType->isPointerType() || ParamType->isReferenceType() ||
@@ -7423,33 +7423,37 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
       // -- a string literal
       // -- the result of a typeid expression, or
       // -- a predefined __func__ variable
-      APValue::LValueBase Base = Value.getLValueBase();
-      auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
       if (Base &&
           (!VD ||
            isa<LifetimeExtendedTemporaryDecl, UnnamedGlobalConstantDecl>(VD))) {
         Diag(Arg->getBeginLoc(), diag::err_template_arg_not_decl_ref)
             << Arg->getSourceRange();
         return ExprError();
       }
-      // -- a subobject [until C++20]
-      if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 &&
-          VD && VD->getType()->isArrayType() &&
+
+      if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 && VD &&
+          VD->getType()->isArrayType() &&
           Value.getLValuePath()[0].getAsArrayIndex() == 0 &&
           !Value.isLValueOnePastTheEnd() && ParamType->isPointerType()) {
-        // Per defect report (no number yet):
-        //   ... other than a pointer to the first element of a complete array
-        //       object.
-      } else if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
-                 Value.isLValueOnePastTheEnd()) {
-        Diag(StartLoc, diag::err_non_type_template_arg_subobject)
-          << Value.getAsString(Context, ParamType);
-        return ExprError();
+        SugaredConverted = TemplateArgument(VD, ParamType);
+        CanonicalConverted = TemplateArgument(
+            cast<ValueDecl>(VD->getCanonicalDecl()), CanonParamType);
+        return ArgResult.get();
+      }
+
+      // -- a subobject [until C++20]
+      if (!getLangOpts().CPlusPlus20) {
+        if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
+            Value.isLValueOnePastTheEnd()) {
+          Diag(StartLoc, diag::err_non_type_template_arg_subobject)
+              << Value.getAsString(Context, ParamType);
+          return ExprError();
+        }
+        assert((VD || !ParamType->isReferenceType()) &&
+               "null reference should not be a constant expression");
+        assert((!VD || !ParamType->isNullPtrType()) &&
+               "non-null value of type nullptr_t?");
       }
-      assert((VD || !ParamType->isReferenceType()) &&
-             "null reference should not be a constant expression");
-      assert((!VD || !ParamType->isNullPtrType()) &&
-             "non-null value of type nullptr_t?");
     }
 
     if (Value.isAddrLabelDiff())

clang/test/CoverageMapping/templates.cpp

@@ -19,3 +19,16 @@ int main() {
   func<bool>(true);
   return 0;
 }
+
+namespace structural_value_crash {
+  template <int* p>
+  void tpl_fn() {
+    (void)p;
+  }
+
+  int arr[] = {1, 2, 3};
+
+  void test() {
+    tpl_fn<arr>();
+  }
+}