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

I found this issue (a separate one) during the investigation of llvm#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.

Author: hokein

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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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;
 

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}}
+}