Adds support to clang"s windows mangler to handle template argument values that are pointers one-past-the-end of a non-array symbol. Also improves error messages in other template argument scenarios where clang bails.

Author: memory-thrasher

clang/lib/AST/MicrosoftMangle.cpp

@@ -1922,11 +1922,19 @@ void MicrosoftCXXNameMangler::mangleTemplateArgValue(QualType T,
     if (WithScalarType)
       mangleType(T, SourceRange(), QMM_Escape);
 
-    // We don't know how to mangle past-the-end pointers yet.
-    if (V.isLValueOnePastTheEnd())
-      break;
-
     APValue::LValueBase Base = V.getLValueBase();
+
+    // this might not cover every case but did cover issue 97756
+    // see test CodeGen/ms_mangler_templatearg_opte
+    if (V.isLValueOnePastTheEnd()) {
+      Out << "5E";
+      auto *VD = Base.dyn_cast<const ValueDecl *>();
+      if (VD)
+        mangle(VD);
+      Out << "@";
+      return;
+    }
+
     if (!V.hasLValuePath() || V.getLValuePath().empty()) {
       // Taking the address of a complete object has a special-case mangling.
       if (Base.isNull()) {
@@ -1938,12 +1946,23 @@ void MicrosoftCXXNameMangler::mangleTemplateArgValue(QualType T,
         mangleNumber(V.getLValueOffset().getQuantity());
       } else if (!V.hasLValuePath()) {
         // FIXME: This can only happen as an extension. Invent a mangling.
-        break;
+        DiagnosticsEngine &Diags = Context.getDiags();
+        unsigned DiagID =
+            Diags.getCustomDiagID(DiagnosticsEngine::Error,
+                                  "cannot mangle this template argument yet "
+                                  "(extension not comaptible with ms mangler)");
+        Diags.Report(DiagID);
+        return;
       } else if (auto *VD = Base.dyn_cast<const ValueDecl*>()) {
         Out << "E";
         mangle(VD);
       } else {
-        break;
+        DiagnosticsEngine &Diags = Context.getDiags();
+        unsigned DiagID = Diags.getCustomDiagID(
+            DiagnosticsEngine::Error,
+            "cannot mangle this template argument yet (undeclared base)");
+        Diags.Report(DiagID);
+        return;
       }
     } else {
       if (TAK == TplArgKind::ClassNTTP && T->isPointerType())
@@ -1988,8 +2007,14 @@ void MicrosoftCXXNameMangler::mangleTemplateArgValue(QualType T,
         Out << *I;
 
       auto *VD = Base.dyn_cast<const ValueDecl*>();
-      if (!VD)
-        break;
+      if (!VD) {
+        DiagnosticsEngine &Diags = Context.getDiags();
+        unsigned DiagID = Diags.getCustomDiagID(
+            DiagnosticsEngine::Error,
+            "cannot mangle this template argument yet (null value decl)");
+        Diags.Report(DiagID);
+        return;
+      }
       Out << (TAK == TplArgKind::ClassNTTP ? 'E' : '1');
       mangle(VD);
 
@@ -2104,15 +2129,24 @@ void MicrosoftCXXNameMangler::mangleTemplateArgValue(QualType T,
     return;
   }
 
-  case APValue::AddrLabelDiff:
-  case APValue::FixedPoint:
-    break;
+  case APValue::AddrLabelDiff: {
+    DiagnosticsEngine &Diags = Context.getDiags();
+    unsigned DiagID = Diags.getCustomDiagID(
+        DiagnosticsEngine::Error, "cannot mangle this template argument yet "
+                                  "(value type: address label diff)");
+    Diags.Report(DiagID);
+    return;
   }
 
-  DiagnosticsEngine &Diags = Context.getDiags();
-  unsigned DiagID = Diags.getCustomDiagID(
-      DiagnosticsEngine::Error, "cannot mangle this template argument yet");
-  Diags.Report(DiagID);
+  case APValue::FixedPoint: {
+    DiagnosticsEngine &Diags = Context.getDiags();
+    unsigned DiagID = Diags.getCustomDiagID(
+        DiagnosticsEngine::Error,
+        "cannot mangle this template argument yet (value type: fixed point)");
+    Diags.Report(DiagID);
+    return;
+  }
+  }
 }
 
 void MicrosoftCXXNameMangler::mangleObjCProtocol(const ObjCProtocolDecl *PD) {

clang/test/CodeGen/ms_mangler_templatearg_opte.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-pc-windows-msvc -emit-llvm -std=c++20 -x c++ < %s | FileCheck -check-prefix=WIN64 %s
+
+struct A {
+  const int* ptr;
+};
+
+template<A> void tfn() {};
+
+// WIN64: ??$tfn@$2UA@@PEBH5CE?ints@@3QBHB06@@@@@YAXXZ
+constexpr int ints[] = { 1, 2, 7, 8, 9, -17, -10 }; //Note: this does NOT break the unpatched mangler
+
+// WIN64: ??$tfn@$2UA@@PEBH5E?one_int@@3HB@@@@YAXXZ
+constexpr int one_int = 7;
+
+void template_instance() {
+  tfn<A{ints + sizeof(ints)/sizeof(int)}>();
+  tfn<A{&one_int + 1}>();
+}
+