From 6acdf58919d5e66809555acf5809b12c54ba79dd Mon Sep 17 00:00:00 2001
From: Emilia Dreamer <emilia@rymiel.space>
Date: Fri, 17 Mar 2023 10:13:02 +0200
Subject: [PATCH] [clang] Properly parse variable template requires clause in
 lambda

Since P0857, part of C++20, a *lambda-expression* can contain a
*requires-clause* after its *template-parameter-list*.

While support for this was added as part of
eccc734a69c0c012ae3160887b65a535b35ead3e, one specific case isn't
handled properly, where the *requires-clause* consists of an
instantiation of a boolean variable template. This is due to a
diagnostic check which was written with the assumption that a
*requires-clause* can never be followed by a left parenthesis. This
assumption no longer holds for lambdas.

This diagnostic check would then attempt to perform a "recovery", but it
does so in a valid parse state, resulting in an invalid parse state
instead!

This patch adds a special case when parsing requires clauses of lambda
templates, to skip this diagnostic check.

Fixes https://github.com/llvm/llvm-project/issues/61278
Fixes https://github.com/llvm/llvm-project/issues/61387

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D146140
---
 clang/docs/ReleaseNotes.rst          |  3 +++
 clang/lib/Sema/SemaConcept.cpp       | 50 +++++++++++++++++++++---------------
 clang/test/SemaTemplate/concepts.cpp |  5 ++++
 3 files changed, 37 insertions(+), 21 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 24f5b62..53001f65 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -80,6 +80,9 @@ C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Support for out-of-line definitions of constrained templates has been improved.
   This partially fixes `#49620 <https://github.com/llvm/llvm-project/issues/49620>`_.
+- Lambda templates with a requires clause directly after the template parameters now parse
+  correctly if the requires clause consists of a variable with a dependent type.
+  (`#61278 <https://github.com/llvm/llvm-project/issues/61278>`_)
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 4a79522..4ff8684 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -105,27 +105,35 @@ bool Sema::CheckConstraintExpression(const Expr *ConstraintExpression,
   QualType Type = ConstraintExpression->getType();
 
   auto CheckForNonPrimary = [&] {
-    if (PossibleNonPrimary)
-      *PossibleNonPrimary =
-          // We have the following case:
-          // template<typename> requires func(0) struct S { };
-          // The user probably isn't aware of the parentheses required around
-          // the function call, and we're only going to parse 'func' as the
-          // primary-expression, and complain that it is of non-bool type.
-          (NextToken.is(tok::l_paren) &&
-           (IsTrailingRequiresClause ||
-            (Type->isDependentType() &&
-             isa<UnresolvedLookupExpr>(ConstraintExpression)) ||
-            Type->isFunctionType() ||
-            Type->isSpecificBuiltinType(BuiltinType::Overload))) ||
-          // We have the following case:
-          // template<typename T> requires size_<T> == 0 struct S { };
-          // The user probably isn't aware of the parentheses required around
-          // the binary operator, and we're only going to parse 'func' as the
-          // first operand, and complain that it is of non-bool type.
-          getBinOpPrecedence(NextToken.getKind(),
-                             /*GreaterThanIsOperator=*/true,
-                             getLangOpts().CPlusPlus11) > prec::LogicalAnd;
+    if (!PossibleNonPrimary)
+      return;
+
+    *PossibleNonPrimary =
+        // We have the following case:
+        // template<typename> requires func(0) struct S { };
+        // The user probably isn't aware of the parentheses required around
+        // the function call, and we're only going to parse 'func' as the
+        // primary-expression, and complain that it is of non-bool type.
+        //
+        // However, if we're in a lambda, this might also be:
+        // []<typename> requires var () {};
+        // Which also looks like a function call due to the lambda parentheses,
+        // but unlike the first case, isn't an error, so this check is skipped.
+        (NextToken.is(tok::l_paren) &&
+         (IsTrailingRequiresClause ||
+          (Type->isDependentType() &&
+           isa<UnresolvedLookupExpr>(ConstraintExpression) &&
+           !dyn_cast_if_present<LambdaScopeInfo>(getCurFunction())) ||
+          Type->isFunctionType() ||
+          Type->isSpecificBuiltinType(BuiltinType::Overload))) ||
+        // We have the following case:
+        // template<typename T> requires size_<T> == 0 struct S { };
+        // The user probably isn't aware of the parentheses required around
+        // the binary operator, and we're only going to parse 'func' as the
+        // first operand, and complain that it is of non-bool type.
+        getBinOpPrecedence(NextToken.getKind(),
+                           /*GreaterThanIsOperator=*/true,
+                           getLangOpts().CPlusPlus11) > prec::LogicalAnd;
   };
 
   // An atomic constraint!
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index df4db9b..bf5896a 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -55,10 +55,15 @@ namespace PR45699 {
 }
 
 namespace P0857R0 {
+  template <typename T> static constexpr bool V = true;
+
   void f() {
     auto x = []<bool B> requires B {}; // expected-note {{constraints not satisfied}} expected-note {{false}}
     x.operator()<true>();
     x.operator()<false>(); // expected-error {{no matching member function}}
+
+    auto y = []<typename T> requires V<T> () {};
+    y.operator()<int>(); // OK
   }
 
   template<typename T> concept C = true;
-- 
2.7.4