[clang] [clang] Fix an out-of-bound crash when checking template partial specializations. (PR #86794)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

<details>
<summary>Changes</summary>

I found this issue (a separate one) during the investigation of #<!-- -->86757, the crash is similar in substituteParameterMappings, but at different inner places.

This was an out-of-bound issue where we access front element in an empty written template argument list to get the instantiation source range. This patch fixes it by adding a proper guard.

---
Full diff: https://github.com/llvm/llvm-project/pull/86794.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+1) 
- (modified) clang/lib/Sema/SemaConcept.cpp (+10-2) 
- (modified) clang/test/SemaTemplate/concepts.cpp (+9-1) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0dd026a5de5c6f..0fdd9e3fb3eee2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -454,6 +454,7 @@ Bug Fixes to C++ Support
 - Fix a crash when instantiating a lambda that captures ``this`` outside of its context. Fixes (#GH85343).
 - Fix an issue where a namespace alias could be defined using a qualified name (all name components
   following the first `::` were ignored).
+- Fix an out-of-bounds crash when checking the validity of template partial specializations. (part of #GH86757).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 1c546e9f5894f0..b6c4d3d540ef50 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1269,10 +1269,18 @@ substituteParameterMappings(Sema &S, NormalizedConstraint &N,
                     : SourceLocation()));
     Atomic.ParameterMapping.emplace(TempArgs,  OccurringIndices.count());
   }
+  SourceLocation InstLocBegin =
+      ArgsAsWritten->arguments().empty()
+          ? ArgsAsWritten->getLAngleLoc()
+          : ArgsAsWritten->arguments().front().getSourceRange().getBegin();
+  SourceLocation InstLocEnd =
+      ArgsAsWritten->arguments().empty()
+          ? ArgsAsWritten->getRAngleLoc()
+          : ArgsAsWritten->arguments().front().getSourceRange().getEnd();
   Sema::InstantiatingTemplate Inst(
-      S, ArgsAsWritten->arguments().front().getSourceRange().getBegin(),
+      S, InstLocBegin,
       Sema::InstantiatingTemplate::ParameterMappingSubstitution{}, Concept,
-      ArgsAsWritten->arguments().front().getSourceRange());
+      {InstLocBegin, InstLocEnd});
   if (S.SubstTemplateArguments(*Atomic.ParameterMapping, MLTAL, SubstArgs))
     return true;
 
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index b7ea0d003a52d7..787cc809e25353 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++20 -verify %s
+// RUN: %clang_cc1 -std=c++20 -ferror-limit 0 -verify %s
 
 namespace PR47043 {
   template<typename T> concept True = true;
@@ -1114,3 +1114,11 @@ void foo() {
 }
 
 } // namespace GH64808
+
+namespace GH86757_1 {
+template <typename...> concept b = false;
+template <typename> concept c = b<>;
+template <typename d> concept f = c< d >;
+template <f> struct e; // expected-note {{}}
+template <f d> struct e<d>; // expected-error {{class template partial specialization is not more specialized than the primary template}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/86794