[Clang] Fix dependent local class instantiation bugs (llvm#134038)

This patch fixes two long-standing bugs that prevent Clang from
instantiating local class members inside a dependent context. These bugs
were introduced in commits 21eb1af and 919df9d.

21eb1af introduced a concept called eligible methods such that it
did an attempt to skip past ineligible method instantiation when
instantiating class members. Unfortunately, this broke the instantiation
chain for local classes - getTemplateInstantiationPattern() would fail
to find the correct definition pattern if the class was defined within a
partially transformed dependent context.

919df9d introduced a separate issue by incorrectly copying the
DeclarationNameInfo during function definition instantiation from the
template pattern, even though that DNI might contain a transformed
TypeSourceInfo. Since that TSI was already updated when the declaration
was instantiated, this led to inconsistencies. As a result, the final
instantiated function could lose track of the transformed declarations,
hence we crash: https://compiler-explorer.com/z/vjvoG76Tf.

This PR corrects them by

1. Removing the bypass logic for method instantiation. The eligible flag
is independent of instantiation and can be updated properly afterward,
so skipping instantiation is unnecessary.

2. Carefully handling TypeSourceInfo by creating a new instance that
preserves the pattern's source location while using the already
transformed type.

Fixes llvm#59734
Fixes llvm#132208

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -434,6 +434,7 @@ Bug Fixes to C++ Support
   by template argument deduction.
 - Clang is now better at instantiating the function definition after its use inside
   of a constexpr lambda. (#GH125747)
+- Fixed a local class member function instantiation bug inside dependent lambdas. (#GH59734), (#GH132208)
 - Clang no longer crashes when trying to unify the types of arrays with
   certain differences in qualifiers (this could happen during template argument
   deduction or when building a ternary operator). (#GH97005)

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -4126,9 +4126,6 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
       if (FunctionDecl *Pattern =
               Function->getInstantiatedFromMemberFunction()) {
 
-        if (Function->isIneligibleOrNotSelected())
-          continue;
-
         if (Function->getTrailingRequiresClause()) {
           ConstraintSatisfaction Satisfaction;
           if (CheckFunctionConstraints(Function, Satisfaction) ||

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -5597,7 +5597,61 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
   Function->setLocation(PatternDecl->getLocation());
   Function->setInnerLocStart(PatternDecl->getInnerLocStart());
   Function->setRangeEnd(PatternDecl->getEndLoc());
-  Function->setDeclarationNameLoc(PatternDecl->getNameInfo().getInfo());
+  // Let the instantiation use the Pattern's DeclarationNameLoc, due to the
+  // following awkwardness:
+  //
+  //   1. There are out-of-tree users of getNameInfo().getSourceRange(), who
+  //   expect the source range of the instantiated declaration to be set to
+  //   point to the definition.
+  //
+  //   2. That getNameInfo().getSourceRange() might return the TypeLocInfo's
+  //   location it tracked.
+  //
+  //   3. Function might come from an (implicit) declaration, while the pattern
+  //   comes from a definition. In these cases, we need the PatternDecl's source
+  //   location.
+  //
+  // To that end, we need to more or less tweak the DeclarationNameLoc. However,
+  // we can't blindly copy the DeclarationNameLoc from the PatternDecl to the
+  // function, since it contains associated TypeLocs that should have already
+  // been transformed. So, we rebuild the TypeLoc for that purpose. Technically,
+  // we should create a new function declaration and assign everything we need,
+  // but InstantiateFunctionDefinition updates the declaration in place.
+  auto NameLocPointsToPattern = [&] {
+    DeclarationNameInfo PatternName = PatternDecl->getNameInfo();
+    DeclarationNameLoc PatternNameLoc = PatternName.getInfo();
+    switch (PatternName.getName().getNameKind()) {
+    case DeclarationName::CXXConstructorName:
+    case DeclarationName::CXXDestructorName:
+    case DeclarationName::CXXConversionFunctionName:
+      break;
+    default:
+      // Cases where DeclarationNameLoc doesn't matter, as it merely contains a
+      // source range.
+      return PatternNameLoc;
+    }
+
+    TypeSourceInfo *TSI = Function->getNameInfo().getNamedTypeInfo();
+    // TSI might be null if the function is named by a constructor template id.
+    // E.g. S<T>() {} for class template S with a template parameter T.
+    if (!TSI) {
+      // We don't care about the DeclarationName of the instantiated function,
+      // but only the DeclarationNameLoc. So if the TypeLoc is absent, we do
+      // nothing.
+      return PatternNameLoc;
+    }
+
+    QualType InstT = TSI->getType();
+    // We want to use a TypeLoc that reflects the transformed type while
+    // preserving the source location from the pattern.
+    TypeLocBuilder TLB;
+    TLB.pushTrivial(
+        Context, InstT,
+        PatternNameLoc.getNamedTypeInfo()->getTypeLoc().getBeginLoc());
+    return DeclarationNameLoc::makeNamedTypeLoc(
+        TLB.getTypeSourceInfo(Context, InstT));
+  };
+  Function->setDeclarationNameLoc(NameLocPointsToPattern());
 
   EnterExpressionEvaluationContext EvalContext(
       *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);

clang/test/CodeGenCXX/local-class-instantiation.cpp

@@ -0,0 +1,64 @@
+// RUN: %clang_cc1 -std=c++17 %s -emit-llvm -triple %itanium_abi_triple -o - | FileCheck %s
+
+namespace LambdaContainingLocalClasses {
+
+template <typename F>
+void GH59734() {
+  [&](auto param) {
+    struct Guard {
+      Guard() {
+        // Check that we're able to create DeclRefExpr to param at this point.
+        static_assert(__is_same(decltype(param), int), "");
+      }
+      ~Guard() {
+        static_assert(__is_same(decltype(param), int), "");
+      }
+      operator decltype(param)() {
+        return decltype(param)();
+      }
+    };
+    Guard guard;
+    param = guard;
+  }(42);
+}
+
+// Guard::Guard():
+// CHECK-DAG: define {{.*}} @_ZZZN28LambdaContainingLocalClasses7GH59734IiEEvvENKUlT_E_clIiEEDaS1_EN5GuardC2Ev
+// Guard::operator int():
+// CHECK-DAG: define {{.*}} @_ZZZN28LambdaContainingLocalClasses7GH59734IiEEvvENKUlT_E_clIiEEDaS1_EN5GuardcviEv
+// Guard::~Guard():
+// CHECK-DAG: define {{.*}} @_ZZZN28LambdaContainingLocalClasses7GH59734IiEEvvENKUlT_E_clIiEEDaS1_EN5GuardD2Ev
+
+struct S {};
+
+template <class T = void>
+auto GH132208 = [](auto param) {
+  struct OnScopeExit {
+    OnScopeExit() {
+      static_assert(__is_same(decltype(param), S), "");
+    }
+    ~OnScopeExit() {
+      static_assert(__is_same(decltype(param), S), "");
+    }
+    operator decltype(param)() {
+      return decltype(param)();
+    }
+  } pending;
+
+  param = pending;
+};
+
+void bar() {
+  GH59734<int>();
+
+  GH132208<void>(S{});
+}
+
+// OnScopeExit::OnScopeExit():
+// CHECK-DAG: define {{.*}} @_ZZNK28LambdaContainingLocalClasses8GH132208IvEMUlT_E_clINS_1SEEEDaS2_EN11OnScopeExitC2Ev
+// OnScopeExit::operator S():
+// CHECK-DAG: define {{.*}} @_ZZNK28LambdaContainingLocalClasses8GH132208IvEMUlT_E_clINS_1SEEEDaS2_EN11OnScopeExitcvS5_Ev
+// OnScopeExit::~OnScopeExit():
+// CHECK-DAG: define {{.*}} @_ZZNK28LambdaContainingLocalClasses8GH132208IvEMUlT_E_clINS_1SEEEDaS2_EN11OnScopeExitD2Ev
+
+} // namespace LambdaContainingLocalClasses