[clang][sema] Provide better diagnostic for missing template parameters

[tbaeder]

Summary

This fixes https://github.com/llvm/llvm-project/issues/55962

i.e. instead of the output being

./test.cpp:5:9: error: 'x' is not a class, namespace, or enumeration
int y = x::a;
        ^
./test.cpp:2:8: note: 'x' declared here
struct x;
       ^
1 error generated.

it is now:

./test.cpp:5:9: error: use of class template 'x' requires template arguments
int y = x::a;
        ^
./test.cpp:2:8: note: template is declared here
struct x;
       ^
1 error generated.

Note that the call to isTemplateName() is basically copied from DiagnoseUnknownTypeName() in SemaDecl.cpp. However, just calling that from SemaCXXScopeSpec.cpp as well makes the SemaObjCXX/property-dot-error.mm test fail, because it now outputs

clang/test/SemaObjCXX/propert-dot-error.mm Line 67: unknown type name 'D'; did you mean 'D'?
clang/test/SemaObjCXX/propert-dot-error.mm Line 68: unknown type name 'D'; did you mean 'D'?

and I don't know enough about objc to fix this, but if anyone has some pointers I'd be happy to avoid the code duplication.

[aaron.ballman]

Note that the call to isTemplateName() is basically copied from DiagnoseUnknownTypeName() in SemaDecl.cpp. However, just calling that from SemaCXXScopeSpec.cpp as well makes the SemaObjCXX/property-dot-error.mm test fail, because it now outputs
clang/test/SemaObjCXX/propert-dot-error.mm Line 67: unknown type name 'D'; did you mean 'D'?
clang/test/SemaObjCXX/propert-dot-error.mm Line 68: unknown type name 'D'; did you mean 'D'?
and I don't know enough about objc to fix this, but if anyone has some pointers I'd be happy to avoid the code duplication.

Perhaps one way around that is to test else if (TemplateDecl *TD = Found.getAsSingle<TemplateDecl>()) {} before calling DiagnoseUnknownTypeName()?

[tbaeder]

That does indeed work, thanks for the suggestion!

[aaron.ballman]

LGTM, thanks for the fix! Please add a release note when you land the changes. :-)

clang/docs/ReleaseNotes.rst

- Unknown preprocessor directives in a skipped conditional block are now given
   suggested when in C2x or C++2b mode. Fixes
   `Issue 51598 <https://github.com/llvm/llvm-project/issues/51598>`_.
 - The ``-Wdeprecated`` diagnostic will now warn on out-of-line ``constexpr``
   declarations downgraded to definitions in C++1z, in addition to the
   existing warning on out-of-line ``const`` declarations.
 - ``-Wshift-overflow`` will not warn for signed left shifts in C++20 mode
   (and newer), as it will always wrap and never overflow. This fixes
   `Issue 52873 <https://github.com/llvm/llvm-project/issues/52873>`_.
-
+- When using class templates without arguments, clang now tells developers
+  that template arguments are missing in certain contexts.
+  This fixes `Issue 55962 <https://github.com/llvm/llvm-project/issues/55962>`_.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
 - Improve __builtin_dump_struct:
 
   - Support bitfields in struct and union.
   - Improve the dump format, dump both bitwidth(if its a bitfield) and field

clang/lib/Sema/SemaCXXScopeSpec.cpp

if (DC->isDependentContext() && DC->isFunctionOrMethod()) {
         SS.Extend(Context, IdInfo.Identifier, IdInfo.IdentifierLoc,
                   IdInfo.CCLoc);
         return false;
       }
     }
   }
 
   if (!Found.empty()) {
-    if (TypeDecl *TD = Found.getAsSingle<TypeDecl>())
+    if (TypeDecl *TD = Found.getAsSingle<TypeDecl>()) {
       Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
           << Context.getTypeDeclType(TD) << getLangOpts().CPlusPlus;
-    else {
+    } else if (Found.getAsSingle<TemplateDecl>()) {
+      ParsedType SuggestedType;
+      DiagnoseUnknownTypeName(IdInfo.Identifier, IdInfo.IdentifierLoc, S, &SS,
+                              SuggestedType);
+    } else {
       Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
           << IdInfo.Identifier << getLangOpts().CPlusPlus;
       if (NamedDecl *ND = Found.getAsSingle<NamedDecl>())
         Diag(ND->getLocation(), diag::note_entity_declared_at)
             << IdInfo.Identifier;
     }
   } else if (SS.isSet())
     Diag(IdInfo.IdentifierLoc, diag::err_no_member) << IdInfo.Identifier

clang/test/SemaCXX/nested-name-spec.cpp

namespace DependentTemplateInTrivialNNSLoc {
   //
   // Previously, creating a CXXScopeSpec from that resulted in an invalid scope
   // spec, leading to crashes. Ensure we don't crash here.
   template <typename T> void f(T &x) {
     for (typename T::template g<> i : x) {} // expected-warning 0-1{{extension}}
     x: goto x;
   }
 }
+
+template <typename T>
+struct x; // expected-note {{template is declared here}}
+
+template <typename T>
+int issue55962 = x::a; // expected-error {{use of class template 'x' requires template arguments}} \
+                       // expected-warning {{variable templates are a C++14 extension}}