[clang] fix a crash in error recovery in expressions resolving to templates (#135893)

We were using AssumedTemplate incorrectly for error recovery.

Fixes #135621

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -402,6 +402,7 @@ Bug Fixes in This Version
   when using the ``INTn_C`` macros. (#GH85995)
 - Fixed an assertion failure in the expansion of builtin macros like ``__has_embed()`` with line breaks before the
   closing paren. (#GH133574)
+- Fixed a crash in error recovery for expressions resolving to templates. (#GH135621)
 - Clang no longer accepts invalid integer constants which are too large to fit
   into any (standard or extended) integer type when the constant is unevaluated.
   Merely forming the token is sufficient to render the program invalid. Code
@@ -544,7 +545,7 @@ Arm and AArch64 Support
 ^^^^^^^^^^^^^^^^^^^^^^^
 - For ARM targets, cc1as now considers the FPU's features for the selected CPU or Architecture.
 - The ``+nosimd`` attribute is now fully supported for ARM. Previously, this had no effect when being used with
-  ARM targets, however this will now disable NEON instructions being generated. The ``simd`` option is 
+  ARM targets, however this will now disable NEON instructions being generated. The ``simd`` option is
   also now printed when the ``--print-supported-extensions`` option is used.
 
 -  Support for __ptrauth type qualifier has been added.

clang/lib/AST/Type.cpp

@@ -4401,7 +4401,8 @@ TemplateSpecializationType::TemplateSpecializationType(
           T.getKind() == TemplateName::SubstTemplateTemplateParmPack ||
           T.getKind() == TemplateName::UsingTemplate ||
           T.getKind() == TemplateName::QualifiedTemplate ||
-          T.getKind() == TemplateName::DeducedTemplate) &&
+          T.getKind() == TemplateName::DeducedTemplate ||
+          T.getKind() == TemplateName::AssumedTemplate) &&
          "Unexpected template name for TemplateSpecializationType");
 
   auto *TemplateArgs =

clang/lib/Sema/SemaExpr.cpp

@@ -21111,11 +21111,15 @@ ExprResult Sema::CheckPlaceholderExpr(Expr *E) {
     const bool IsTypeAliasTemplateDecl = isa<TypeAliasTemplateDecl>(Temp);
 
     NestedNameSpecifier *NNS = ULE->getQualifierLoc().getNestedNameSpecifier();
-    TemplateName TN(dyn_cast<TemplateDecl>(Temp));
-    if (TN.isNull())
+    // FIXME: AssumedTemplate is not very appropriate for error recovery here,
+    // as it models only the unqualified-id case, where this case can clearly be
+    // qualified. Thus we can't just qualify an assumed template.
+    TemplateName TN;
+    if (auto *TD = dyn_cast<TemplateDecl>(Temp))
+      TN = Context.getQualifiedTemplateName(NNS, ULE->hasTemplateKeyword(),
+                                            TemplateName(TD));
+    else
       TN = Context.getAssumedTemplateName(NameInfo.getName());
-    TN = Context.getQualifiedTemplateName(NNS,
-                                          /*TemplateKeyword=*/true, TN);
 
     Diag(NameInfo.getLoc(), diag::err_template_kw_refers_to_type_template)
         << TN << ULE->getSourceRange() << IsTypeAliasTemplateDecl;

clang/test/SemaTemplate/recovery-crash.cpp

@@ -67,3 +67,14 @@ namespace test1 {
   // expected-note@#defined-here {{defined here}}
   void NonTemplateClass::UndeclaredMethod() {}
 }
+
+namespace GH135621 {
+  template <class T> struct S {};
+  // expected-note@-1 {{class template declared here}}
+  template <class T2> void f() {
+    S<T2>::template S<int>;
+    // expected-error@-1 {{'S' is expected to be a non-type template, but instantiated to a class template}}
+  }
+  template void f<int>();
+  // expected-note@-1 {{requested here}}
+} // namespace GH135621