[clang] Add frontend flag to enable support for broken external resugarers

clang/include/clang/Basic/LangOptions.def

@@ -162,6 +162,7 @@ LANGOPT(CoroAlignedAllocation, 1, 0, "prefer Aligned Allocation according to P20
 LANGOPT(DllExportInlines  , 1, 1, "dllexported classes dllexport inline methods")
 LANGOPT(RelaxedTemplateTemplateArgs, 1, 1, "C++17 relaxed matching of template template arguments")
 LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library features")
+LANGOPT(RetainSubstTemplateTypeParmTypeAstNodes, 1, 0, "retain SubstTemplateTypeParmType nodes in the AST's representationof alias template specializations")
 
 LANGOPT(PointerAuthIntrinsics, 1, 0, "pointer authentication intrinsics")
 LANGOPT(PointerAuthCalls  , 1, 0, "function pointer authentication")

clang/include/clang/Driver/Options.td

@@ -3455,6 +3455,12 @@ defm relaxed_template_template_args : BoolFOption<"relaxed-template-template-arg
   PosFlag<SetTrue, [], [], "Enable">,
   NegFlag<SetFalse, [], [CC1Option], "Disable">,
   BothFlags<[], [ClangOption], " C++17 relaxed template template argument matching">>;
+defm retain_subst_template_type_parm_type_ast_nodes : BoolFOption<"retain-subst-template-type-parm-type-ast-nodes",
+  LangOpts<"RetainSubstTemplateTypeParmTypeAstNodes">, DefaultFalse,
+  PosFlag<SetTrue, [], [CC1Option], "Enable">,
+  NegFlag<SetFalse, [], [], "Disable">,
+  BothFlags<[], [], " retain SubstTemplateTypeParmType nodes in the AST's representation"
+  " of alias template specializations">>;
 defm sized_deallocation : BoolFOption<"sized-deallocation",
   LangOpts<"SizedDeallocation">, Default<cpp14.KeyPath>,
   PosFlag<SetTrue, [], [], "Enable C++14 sized global deallocation functions">,

clang/lib/Sema/SemaTemplate.cpp

@@ -3332,10 +3332,16 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Pattern->isInvalidDecl())
       return QualType();
 
-    // Only substitute for the innermost template argument list.
+    // Only substitute for the innermost template argument list.  NOTE: Some
+    // external resugarers rely on leaving a Subst* node here.  Make the
+    // substitution non-final in that case.  Note that these external resugarers
+    // will still miss some information in this representation, because we don't
+    // provide enough context in the Subst* nodes in order to tell different
+    // template type alias specializations apart.
     MultiLevelTemplateArgumentList TemplateArgLists;
-    TemplateArgLists.addOuterTemplateArguments(Template, SugaredConverted,
-                                               /*Final=*/true);
+    TemplateArgLists.addOuterTemplateArguments(
+        Template, SugaredConverted,
+        /*Final=*/!getLangOpts().RetainSubstTemplateTypeParmTypeAstNodes);
     TemplateArgLists.addOuterRetainedLevels(
         AliasTemplate->getTemplateParameters()->getDepth());
 

clang/test/AST/ast-dump-support-broken-external-resugarers.cpp

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -fsyntax-only -fretain-subst-template-type-parm-type-ast-nodes -ast-dump -ast-dump-filter=dump %s | FileCheck -strict-whitespace %s
+
+namespace t1 {
+template<class T> using X = T;
+using dump = X<int>;
+
+// CHECK-LABEL: Dumping t1::dump:
+// CHECK-NEXT:  TypeAliasDecl
+// CHECK-NEXT:  `-ElaboratedType
+// CHECK-NEXT:    `-TemplateSpecializationType
+// CHECK-NEXT:      |-name: 'X':'t1::X' qualified
+// CHECK-NEXT:      | `-TypeAliasTemplateDecl
+// CHECK-NEXT:      |-TemplateArgument
+// CHECK-NEXT:      | `-BuiltinType {{.+}} 'int'
+// CHECK-NEXT:      `-SubstTemplateTypeParmType 0x{{[0-9]+}} 'int' sugar class depth 0 index 0 T
+// CHECK-NEXT:        |-TypeAliasTemplate {{.+}} 'X'
+// CHECK-NEXT:        `-BuiltinType {{.+}} 'int'
+} // namespace t1