[clang] [Clang] Keep unused template parameters in MLTAL with empty packs (PR #174698)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

Unused template parameters, though never referenced during substitution, must remain in the MLTAL to preserve consistent template parameter indices.

The null TemplateArgument played this role before, which caused a crash due to existing assumptions in computing depenedency, so an empty pack is used instead.

There is no release note because this is a regression from concept parameter mapping.

Fixes https://github.com/llvm/llvm-project/issues/174667

---
Full diff: https://github.com/llvm/llvm-project/pull/174698.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaConcept.cpp (+8-1) 
- (modified) clang/test/SemaTemplate/concepts.cpp (+10) 


``````````diff
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 2674f0a7b8749..a52cebe5a62c2 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -639,7 +639,14 @@ ConstraintSatisfactionChecker::SubstitutionInTemplateArguments(
         llvm::to_vector_of<TemplateArgument>(MLTAL.getOutermost());
   unsigned Offset = 0;
   for (unsigned I = 0, MappedIndex = 0; I < Used.size(); I++) {
-    TemplateArgument Arg;
+    // For template parameters that are not used during substitution, they
+    // still need to exist in the MLTAL since template parameter indices remain
+    // unchanged.
+    // We need a placeholder to represent them: a null TemplateArgument doesn't
+    // fit well due to non-null assumptions elsewhere. An empty pack serves this
+    // purpose, and substitution checks would catch any mistakes if we really
+    // messed anything up.
+    TemplateArgument Arg = TemplateArgument::getEmptyPack();
     if (Used[I])
       Arg = S.Context.getCanonicalTemplateArgument(
           CTAI.SugaredConverted[MappedIndex++]);
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index eed8b786f9954..d93391baf9926 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -1703,3 +1703,13 @@ struct : named_unit<"", thermodynamic_temperature, zeroth_kelvin> {
 struct ice_point : relative_point_origin<point<kelvin>> {};
 
 }
+
+namespace GH174667 {
+
+template<class T, class, class U>
+concept C = requires{ requires U(T(1)); };
+
+template<C<void, bool> T> int f();
+void main() { f<int>(); }
+
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/174698