[Clang][Sema] Tweak tryCaptureVariable for unevaluated lambdas (#93206)

This patch picks up #78598 with the hope that we can address such
crashes in `tryCaptureVariable()` for unevaluated lambdas.

In addition to `tryCaptureVariable()`, this also contains several other
fixes on e.g. lambda parsing/dependencies.

Fixes #63845
Fixes #67260
Fixes #69307
Fixes #88081
Fixes #89496
Fixes #90669
Fixes #91633

clang/docs/ReleaseNotes.rst

@@ -824,6 +824,8 @@ Bug Fixes to C++ Support
 - Fix a crash when a variable is captured by a block nested inside a lambda. (Fixes #GH93625).
 - Fixed a type constraint substitution issue involving a generic lambda expression. (#GH93821)
 - Fix a crash caused by improper use of ``__array_extent``. (#GH80474)
+- Fixed several bugs in capturing variables within unevaluated contexts. (#GH63845), (#GH67260), (#GH69307),
+  (#GH88081), (#GH89496), (#GH90669) and (#GH91633).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/DeclBase.h

@@ -2148,6 +2148,10 @@ class DeclContext {
            getDeclKind() <= Decl::lastRecord;
   }
 
+  bool isRequiresExprBody() const {
+    return getDeclKind() == Decl::RequiresExprBody;
+  }
+
   bool isNamespace() const { return getDeclKind() == Decl::Namespace; }
 
   bool isStdNamespace() const;

clang/lib/Parse/ParseExprCXX.cpp

@@ -1581,7 +1581,10 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
                       TrailingReturnTypeLoc, &DS),
                   std::move(Attributes), DeclEndLoc);
 
-    Actions.ActOnLambdaClosureQualifiers(Intro, MutableLoc);
+    // We have called ActOnLambdaClosureQualifiers for parentheses-less cases
+    // above.
+    if (HasParentheses)
+      Actions.ActOnLambdaClosureQualifiers(Intro, MutableLoc);
 
     if (HasParentheses && Tok.is(tok::kw_requires))
       ParseTrailingRequiresClause(D);

clang/lib/Sema/SemaExpr.cpp

@@ -18768,13 +18768,23 @@ bool Sema::tryCaptureVariable(
   DeclContext *VarDC = Var->getDeclContext();
   DeclContext *DC = CurContext;
 
+  // Skip past RequiresExprBodys because they don't constitute function scopes.
+  while (DC->isRequiresExprBody())
+    DC = DC->getParent();
+
   // tryCaptureVariable is called every time a DeclRef is formed,
   // it can therefore have non-negigible impact on performances.
   // For local variables and when there is no capturing scope,
   // we can bailout early.
   if (CapturingFunctionScopes == 0 && (!BuildAndDiagnose || VarDC == DC))
     return true;
 
+  // Exception: Function parameters are not tied to the function's DeclContext
+  // until we enter the function definition. Capturing them anyway would result
+  // in an out-of-bounds error while traversing DC and its parents.
+  if (isa<ParmVarDecl>(Var) && !VarDC->isFunctionOrMethod())
+    return true;
+
   const auto *VD = dyn_cast<VarDecl>(Var);
   if (VD) {
     if (VD->isInitCapture())

clang/lib/Sema/SemaLambda.cpp

@@ -1076,16 +1076,27 @@ void Sema::ActOnLambdaExpressionAfterIntroducer(LambdaIntroducer &Intro,
   // be dependent, because there are template parameters in scope.
   CXXRecordDecl::LambdaDependencyKind LambdaDependencyKind =
       CXXRecordDecl::LDK_Unknown;
-  if (LSI->NumExplicitTemplateParams > 0) {
-    Scope *TemplateParamScope = CurScope->getTemplateParamParent();
-    assert(TemplateParamScope &&
-           "Lambda with explicit template param list should establish a "
-           "template param scope");
-    assert(TemplateParamScope->getParent());
-    if (TemplateParamScope->getParent()->getTemplateParamParent() != nullptr)
-      LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
-  } else if (CurScope->getTemplateParamParent() != nullptr) {
+  if (CurScope->getTemplateParamParent() != nullptr) {
     LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
+  } else if (Scope *P = CurScope->getParent()) {
+    // Given a lambda defined inside a requires expression,
+    //
+    // struct S {
+    //   S(auto var) requires requires { [&] -> decltype(var) { }; }
+    //   {}
+    // };
+    //
+    // The parameter var is not injected into the function Decl at the point of
+    // parsing lambda. In such scenarios, perceiving it as dependent could
+    // result in the constraint being evaluated, which matches what GCC does.
+    while (P->getEntity() && P->getEntity()->isRequiresExprBody())
+      P = P->getParent();
+    if (P->isFunctionDeclarationScope() &&
+        llvm::any_of(P->decls(), [](Decl *D) {
+          return isa<ParmVarDecl>(D) &&
+                 cast<ParmVarDecl>(D)->getType()->isTemplateTypeParmType();
+        }))
+      LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
   }
 
   CXXRecordDecl *Class = createLambdaClosureType(

clang/lib/Sema/TreeTransform.h

@@ -14247,7 +14247,7 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   // will be deemed as dependent even if there are no dependent template
   // arguments.
   // (A ClassTemplateSpecializationDecl is always a dependent context.)
-  while (DC->getDeclKind() == Decl::Kind::RequiresExprBody)
+  while (DC->isRequiresExprBody())
     DC = DC->getParent();
   if ((getSema().isUnevaluatedContext() ||
        getSema().isConstantEvaluatedContext()) &&

clang/test/SemaCXX/lambda-unevaluated.cpp

@@ -189,3 +189,78 @@ void recursive() {
 
 }
 }
+
+// GH63845: Test if we have skipped past RequiresExprBodyDecls in tryCaptureVariable().
+namespace GH63845 {
+
+template <bool> struct A {};
+
+struct true_type {
+  constexpr operator bool() noexcept { return true; }
+};
+
+constexpr bool foo() {
+  true_type x{};
+  return requires { typename A<x>; };
+}
+
+static_assert(foo());
+
+} // namespace GH63845
+
+// GH69307: Test if we can correctly handle param decls that have yet to get into the function scope.
+namespace GH69307 {
+
+constexpr auto ICE() {
+  constexpr auto b = 1;
+  return [=](auto c) -> int
+           requires requires { b + c; }
+  { return 1; };
+};
+
+constexpr auto Ret = ICE()(1);
+
+} // namespace GH69307
+
+// GH88081: Test if we evaluate the requires expression with lambda captures properly.
+namespace GH88081 {
+
+// Test that ActOnLambdaClosureQualifiers() is called only once.
+void foo(auto value)
+  requires requires { [&] -> decltype(value) {}; }
+  // expected-error@-1 {{non-local lambda expression cannot have a capture-default}}
+{}
+
+struct S { //#S
+  S(auto value) //#S-ctor
+  requires requires { [&] -> decltype(value) { return 2; }; } {} // #S-requires
+
+  static auto foo(auto value) -> decltype([&]() -> decltype(value) {}()) { return {}; } // #S-foo
+
+  // FIXME: 'value' does not constitute an ODR use here. Add a diagnostic for it.
+  static auto bar(auto value) -> decltype([&] { return value; }()) {
+    return "a"; // #bar-body
+  }
+};
+
+S s("a"); // #use
+// expected-error@#S-requires {{cannot initialize return object of type 'decltype(value)' (aka 'const char *') with an rvalue of type 'int'}}
+// expected-error@#use {{no matching constructor}}
+// expected-note@#S-requires {{substituting into a lambda expression here}}
+// expected-note@#S-requires {{substituting template arguments into constraint expression here}}
+// expected-note@#S-requires {{in instantiation of requirement here}}
+// expected-note@#use {{checking constraint satisfaction for template 'S<const char *>' required here}}
+// expected-note@#use {{requested here}}
+// expected-note-re@#S 2{{candidate constructor {{.*}} not viable}}
+// expected-note@#S-ctor {{constraints not satisfied}}
+// expected-note-re@#S-requires {{because {{.*}} would be invalid}}
+
+void func() {
+  S::foo(42);
+  S::bar("str");
+  S::bar(0.618);
+  // expected-error-re@#bar-body {{cannot initialize return object of type {{.*}} (aka 'double') with an lvalue of type 'const char[2]'}}
+  // expected-note@-2 {{requested here}}
+}
+
+} // namespace GH88081