[Clang] Fix dependent expression handling for assumptions (llvm#115646)

The function definition instantiation assumes any declarations used
inside are already transformed before transforming the body, so we need
to preserve the transformed expression of CXXAssumeAttr even if it is
not a constant expression. Moreover, the full expression of the
assumption should also entail a potential lambda capture transformation,
hence the call to ActOnFinishFullExpr() after TransformExpr().

Fixes llvm#114787

clang/docs/ReleaseNotes.rst

@@ -652,6 +652,8 @@ Bug Fixes to C++ Support
   an implicitly instantiated class template specialization. (#GH51051)
 - Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
+- Fixed expression transformation for ``[[assume(...)]]``, allowing using pack indexing expressions within the
+  assumption if they also occur inside of a dependent lambda. (#GH114787)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -2196,11 +2196,18 @@ TemplateInstantiator::TransformCXXAssumeAttr(const CXXAssumeAttr *AA) {
   if (!Res.isUsable())
     return AA;
 
-  Res = getSema().BuildCXXAssumeExpr(Res.get(), AA->getAttrName(),
-                                     AA->getRange());
+  Res = getSema().ActOnFinishFullExpr(Res.get(),
+                                      /*DiscardedValue=*/false);
   if (!Res.isUsable())
     return AA;
 
+  if (!(Res.get()->getDependence() & ExprDependence::TypeValueInstantiation)) {
+    Res = getSema().BuildCXXAssumeExpr(Res.get(), AA->getAttrName(),
+                                       AA->getRange());
+    if (!Res.isUsable())
+      return AA;
+  }
+
   return CXXAssumeAttr::CreateImplicit(getSema().Context, Res.get(),
                                        AA->getRange());
 }

clang/test/SemaCXX/cxx23-assume.cpp

@@ -2,6 +2,8 @@
 // RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext,expected
 // RUN: %clang_cc1 -std=c++23  -x c++ %s -verify -fexperimental-new-constant-interpreter
 // RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext,expected -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++26  -x c++ %s -verify
+// RUN: %clang_cc1 -std=c++26  -x c++ %s -verify -fexperimental-new-constant-interpreter
 
 struct A{};
 struct B{ explicit operator bool() { return true; } };
@@ -167,3 +169,29 @@ int foo () {
     __attribute__((assume (a < b)));
 }
 }
+
+namespace GH114787 {
+
+// FIXME: Correct the C++26 value
+#if __cplusplus >= 202400L
+
+constexpr int test(auto... xs) {
+  // FIXME: Investigate why addresses of PackIndexingExprs are printed for the next
+  // 'in call to' note.
+  return [&]<int I>() { // expected-note {{in call to}}
+    [[assume(
+      xs...[I] == 2
+    )]];
+    [[assume(
+      xs...[I + 1] == 0 // expected-note {{assumption evaluated to false}}
+    )]];
+    return xs...[I];
+  }.template operator()<1>();
+}
+
+static_assert(test(1, 2, 3, 5, 6) == 2); // expected-error {{not an integral constant expression}} \
+                                         // expected-note {{in call to}}
+
+#endif
+
+} // namespace GH114787