[clang] [Clang][Sema] Fix exception specification comparison for functions with different template depths (PR #111561)

[Younan Zhang via cfe-commits]

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@@ -501,6 +519,89 @@ bool Sema::CheckEquivalentExceptionSpec(
   return Result;
 }
 
+static const Expr *SubstituteExceptionSpecWithoutEvaluation(
+    Sema &S, const Sema::TemplateCompareNewDeclInfo &DeclInfo,
+    const Expr *ExceptionSpec) {
+  MultiLevelTemplateArgumentList MLTAL = S.getTemplateInstantiationArgs(
+      DeclInfo.getDecl(), DeclInfo.getLexicalDeclContext(),
+      /*Final=*/false, /*Innermost=*/std::nullopt,
+      /*RelativeToPrimary=*/true, /*ForConstraintInstantiation=*/true);
+
+  if (!MLTAL.getNumSubstitutedLevels())
+    return ExceptionSpec;
+
+  Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/false);
+
+  Sema::InstantiatingTemplate Inst(
+      S, DeclInfo.getLocation(),
+      const_cast<FunctionDecl *>(DeclInfo.getDecl()->getAsFunction()),
+      Sema::InstantiatingTemplate::ExceptionSpecification());
+  if (Inst.isInvalid())
+    return nullptr;
+
+  // Set up a dummy 'instantiation' scope in the case of reference to function
+  // parameters that the surrounding function hasn't been instantiated yet. Note
+  // this may happen while we're comparing two templates' constraint
+  // equivalence.
+  LocalInstantiationScope ScopeForParameters(S);
+  if (auto *FD = DeclInfo.getDecl()->getAsFunction())
+    for (auto *PVD : FD->parameters())
+      ScopeForParameters.InstantiatedLocal(PVD, PVD);
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zyn0217 wrote:

This seems different from the latest version of AreConstraintExpressionEquivalent(), which expands parameter packs to a size-of-1 pack argument. Can you explain why there is a difference?

https://github.com/llvm/llvm-project/pull/111561