[clang-tools-extra] [lldb] [clang] [c++20] P1907R1: Support for generalized non-type template arguments of scalar type. (PR #78041)

[Aaron Ballman via cfe-commits]

================
@@ -4833,9 +4833,26 @@ void CXXNameMangler::mangleExpression(const Expr *E, unsigned Arity,
     E = cast<CXXStdInitializerListExpr>(E)->getSubExpr();
     goto recurse;
 
-  case Expr::SubstNonTypeTemplateParmExprClass:
+  case Expr::SubstNonTypeTemplateParmExprClass: {
+    // Mangle a substituted parameter the same way we mangle the template
+    // argument.
+    auto *SNTTPE = cast<SubstNonTypeTemplateParmExpr>(E);
+    if (auto *CE = dyn_cast<ConstantExpr>(SNTTPE->getReplacement())) {
+      // Pull out the constant value and mangle it as a template argument.
+      QualType ParamType = SNTTPE->getParameterType(Context.getASTContext());
+      if (CE->hasAPValueResult())
+        mangleValueInTemplateArg(ParamType, CE->getResultAsAPValue(), false,
+                                 /*NeedExactType=*/true);
+      else
+        mangleValueInTemplateArg(ParamType, CE->getAPValueResult(), false,
+                                 /*NeedExactType=*/true);
----------------
AaronBallman wrote:

```suggestion
      assert(CE->hasAPValueResult() && "expected the NTTP to have an APValue");
      mangleValueInTemplateArg(ParamType, CE->getAPValueResult(), false,
                                 /*NeedExactType=*/true);
```
By the time we get here, we had better have an `APValue` result object already, but also `hasAPValueResult()` is looking at the `APValueKind` bitfield while `getResultAsAPValue()` is checking the `ResultKind` bitfield.

https://github.com/llvm/llvm-project/pull/78041