[clang] Skip re-building lambda expressions in parameters to consteval fns.

[usaxena95]

Summary

As discussed in this comment, we end up building the lambda twice: once while parsing the function calls and then again while handling the immediate invocation.
This happens specially during removing nested immediate invocation. Eg: When we have another consteval function as the parameter along with this lambda expression. Eg: foo(bar([]{})), foo(bar(), []{})

While removing such nested immediate invocations, we should not rebuild this lambda. (IIUC, rebuilding a lambda would always generate a new type which will never match the original type from parsing)

Fixes: https://github.com/llvm/llvm-project/issues/56183
Fixes: https://github.com/llvm/llvm-project/issues/51695
Fixes: https://github.com/llvm/llvm-project/issues/50455
Fixes: https://github.com/llvm/llvm-project/issues/54872
Fixes: https://github.com/llvm/llvm-project/issues/54587

[usaxena95]

More tests.

[usaxena95]

new line.

[usaxena95]

Add some test with expected errors as well.

[usaxena95]

This actually fixes two more issues.
Update release notes and add more documentation.

[usaxena95]

Gentle ping for review.

[ilya-biryukov]

Thanks, that's a nice find!

Could you also add a test that mentions uses consteval functions in lambda captures inside immediate invocations?
Something like consteval_foo([x = consteval_foo(1)]() consteval { return x; }) and maybe also without consteval on lambda parameter list.
I suspect this should work as expected, just wanted to make sure we capture this in tests

clang/docs/ReleaseNotes.rst

219

NIT

clang/lib/Sema/SemaExpr.cpp

17601–17605

NIT: I tried to expand the meaning of term 'built' to make the comment a bit clearer.

[usaxena95]

Addressed comments and added more tests.

clang/docs/ReleaseNotes.rst

- Implemented "Conditionally Trivial Special Member Functions" (`P0848 <https://wg21.link/p0848r3>`_).
   Note: The handling of deleted functions is not yet compliant, as Clang
   does not implement `DR1496 <https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1496>`_
   and `DR1734 <https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1734>`_.
 - Class member variables are now in scope when parsing a ``requires`` clause. Fixes
   `GH55216 <https://github.com/llvm/llvm-project/issues/55216>`_.
 - Correctly set expression evaluation context as 'immediate function context' in
   consteval functions.
   This fixes `GH51182 <https://github.com/llvm/llvm-project/issues/51182>`
+
 - Fixes an assert crash caused by looking up missing vtable information on ``consteval``
   virtual functions. Fixes `GH55065 <https://github.com/llvm/llvm-project/issues/55065>`_.
 
+- Skip rebuilding lambda expressions in arguments of immediate invocations.
+  This fixes `GH56183 <https://github.com/llvm/llvm-project/issues/56183>`_,
+  `GH51695 <https://github.com/llvm/llvm-project/issues/51695>`_,
+  `GH50455 <https://github.com/llvm/llvm-project/issues/50455>`_,
+  `GH54872 <https://github.com/llvm/llvm-project/issues/54872>`_,
+  `GH54587 <https://github.com/llvm/llvm-project/issues/54587>`_.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 CUDA/HIP Language Changes in Clang
 ----------------------------------
 
 Objective-C Language Changes in Clang

clang/lib/Sema/SemaExpr.cpp

ExprResult TransformInitializer(Expr *Init, bool NotCopyInit) {
         if (CE->isImmediateInvocation())
           RemoveImmediateInvocation(CE);
       return Base::TransformInitializer(Init, NotCopyInit);
     }
     ExprResult TransformDeclRefExpr(DeclRefExpr *E) {
       DRSet.erase(E);
       return E;
     }
+    ExprResult TransformLambdaExpr(LambdaExpr *E) {
+      // Do not rebuild lambdas to avoid creating a new type.
+      // Lambdas have already been processed inside their eval context.
+      return E;
+    }
     bool AlwaysRebuild() { return false; }
     bool ReplacingOriginal() { return true; }
     bool AllowSkippingCXXConstructExpr() {
       bool Res = AllowSkippingFirstCXXConstructExpr;
       AllowSkippingFirstCXXConstructExpr = true;
       return Res;
     }
     bool AllowSkippingFirstCXXConstructExpr = true;

clang/test/SemaCXX/cxx2a-consteval.cpp

if (m == a)
         return max(b, c);
     if (m == b)
         return max(a, c);
     return max(a, b);
 }
 static_assert(max(1,2)==2);
 static_assert(mid(1,2,3)==2);
 } // namespace GH51182
+
+// https://github.com/llvm/llvm-project/issues/56183
+namespace GH56183 {
+consteval auto Foo(auto c) { return c; }
+consteval auto Bar(auto f) { return f(); }
+void test() {
+  constexpr auto x = Foo(Bar([] { return 'a'; }));
+  static_assert(x == 'a');
+}
+}  // namespace GH56183
+
+// https://github.com/llvm/llvm-project/issues/51695
+namespace GH51695 {
+// Original ========================================
+template <typename T>
+struct type_t {};
+
+template <typename...>
+struct list_t {};
+
+template <typename T, typename... Ts>
+consteval auto pop_front(list_t<T, Ts...>) -> auto {
+  return list_t<Ts...>{};
+}
+
+template <typename... Ts, typename F>
+consteval auto apply(list_t<Ts...>, F fn) -> auto {
+  return fn(type_t<Ts>{}...);
+}
+
+void test1() {
+  constexpr auto x = apply(pop_front(list_t<char, char>{}),
+                            []<typename... Us>(type_t<Us>...) { return 42; });
+  static_assert(x == 42);
+}
+// Reduced 1 ========================================
+consteval bool zero() { return false; }
+
+template <typename F>
+consteval bool foo(bool, F f) {
+  return f();
+}
+
+void test2() {
+  constexpr auto x = foo(zero(), []() { return true; });
+  static_assert(x);
+}
+
+// Reduced 2 ========================================
+template <typename F>
+consteval auto bar(F f) { return f;}
+
+void test3() {
+  constexpr auto t1 = bar(bar(bar(bar([]() { return true; }))))();
+  static_assert(t1);
+
+  int a = 1; // expected-note {{declared here}}
+  auto t2 = bar(bar(bar(bar([=]() { return a; }))))(); // expected-error-re {{call to consteval function 'GH51695::bar<(lambda at {{.*}})>' is not a constant expression}}
+  // expected-note@-1 {{read of non-const variable 'a' is not allowed in a constant expression}}
+
+  constexpr auto t3 = bar(bar([x=bar(42)]() { return x; }))();
+  static_assert(t3==42);
+  constexpr auto t4 = bar(bar([x=bar(42)]() consteval { return x; }))();
+  static_assert(t4==42);
+}
+
+}  // namespace GH51695
+
+// https://github.com/llvm/llvm-project/issues/50455
+namespace GH50455 {
+void f() {
+  []() consteval { int i{}; }();
+  []() consteval { int i{}; ++i; }();
+}
+void g() {
+  (void)[](int i) consteval { return i; }(0);
+  (void)[](int i) consteval { return i; }(0);
+}
+}  // namespace GH50455