Itanium ABI: Template template parameters are usable as substitutions

[majnemer]

Summary

Template template parameters weren't added to the list of substitutions.
This would make the substitution map contain inaccurate mappings,
leading to Clang violating the Itanium ABI and breaking compatibility
with GCC.

This fixes PR21351.

[rsmith]

LGTM, thanks!

[majnemer]

Closed by commit rL220588 (authored by @majnemer).

lib/AST/ItaniumMangle.cpp

 
 void CXXNameMangler::mangleUnscopedTemplateName(const TemplateDecl *ND) {
   //     <unscoped-template-name> ::= <unscoped-name>
   //                              ::= <substitution>
   if (mangleSubstitution(ND))
     return;
 
   // <template-template-param> ::= <template-param>
-  if (const TemplateTemplateParmDecl *TTP
-                                     = dyn_cast<TemplateTemplateParmDecl>(ND)) {
+  if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND))
     mangleTemplateParameter(TTP->getIndex());
-    return;
-  }
-
+  else
     mangleUnscopedName(ND->getTemplatedDecl());
+
   addSubstitution(ND);
 }
 
 void CXXNameMangler::mangleUnscopedTemplateName(TemplateName Template) {
   //     <unscoped-template-name> ::= <unscoped-name>
   //                              ::= <substitution>
   if (TemplateDecl *TD = Template.getAsTemplateDecl())
     return mangleUnscopedTemplateName(TD);
void CXXNameMangler::mangleTemplatePrefix(const TemplateDecl *ND,
   //                   ::= <substitution>
   // <template-template-param> ::= <template-param>
   //                               <substitution>
 
   if (mangleSubstitution(ND))
     return;
 
   // <template-template-param> ::= <template-param>
-  if (const TemplateTemplateParmDecl *TTP
-                                     = dyn_cast<TemplateTemplateParmDecl>(ND)) {
+  if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(ND)) {
     mangleTemplateParameter(TTP->getIndex());
-    return;
-  }
-
+  } else {
     manglePrefix(getEffectiveDeclContext(ND), NoFunction);
     mangleUnqualifiedName(ND->getTemplatedDecl());
+  }
+
   addSubstitution(ND);
 }
 
 /// Mangles a template name under the production <type>.  Required for
 /// template template arguments.
 ///   <type> ::= <class-enum-type>
 ///          ::= <template-param>
 ///          ::= <substitution>

test/CodeGenCXX/mangle.cpp

namespace test48 {
   struct S {
     union u {};
   };
   template <typename T>
   void f(union T::u *) {}
   template void f<S>(S::u *);
   // CHECK-LABEL: define weak_odr void @_ZN6test481fINS_1SEEEvPTuNT_1uE(%"union.test48::S::u"*)
 }
+
+namespace test49 {
+  template <int>
+  struct S {};
+
+  template <template <int> class T>
+  T<3> fin(T<3>);
+
+  auto v = fin<S>;
+  // CHECK-LABEL: declare void @_ZN6test493finINS_1SEEET_ILi3EES3_()
+}
+
+namespace test50 {
+  template <int>
+  struct S {};
+
+  template <template <int> class T>
+  T<3> fin(T<4>);
+
+  auto v = fin<S>;
+  // CHECK-LABEL: declare void @_ZN6test503finINS_1SEEET_ILi3EES2_ILi4EE()
+}