[Clang] Fix the instantiation of return type requirements in lambda bodies

[LYP951018]

Currently, due to the incomplete implementation of p0588r1, the instantiation of lambda expressions leads to the instantiation of the body. And EvaluateConstraints is false during the instantiation of the body, which causes crashes during the instantiation of the return type requirement:

template<typename T> concept doesnt_matter = true;

template<class T>
concept test = 
    []{
        return requires(T t) {
            { t } -> doesnt_matter; // crash
        };
    }();

static_assert(test<int>);

Although a complete implementation of p0588r1 can solve these crashes, it will take some time. Therefore, this pull request aims to fix these crashes first.

Fixes #63808
Fixes #64607
Fixes #64086

[llvmbot]

@llvm/pr-subscribers-clang

Author: 刘雨培 (LYP951018)

Changes

Currently, due to the incomplete implementation of p0588r1, the instantiation of lambda expressions leads to the instantiation of the body. And EvaluateConstraints is false during the instantiation of the body, which causes crashes during the instantiation of the return type requirement:

template<typename T> concept doesnt_matter = true;

template<class T>
concept test = 
    []{
        return requires(T t) {
            { t } -> doesnt_matter; // crash
        };
    }();

static_assert(test<int>);

Although a complete implementation of p0588r1 can solve these crashes, it will take some time. Therefore, this pull request aims to fix these crashes first.

Fixes #63808, #64607, #64086

---

Full diff: https://github.com/llvm/llvm-project/pull/76967.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+8)
• (modified) clang/test/SemaTemplate/concepts-lambda.cpp (+33)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 30cfe66703f5a9..1815e8d69332e2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -843,6 +843,9 @@ Bug Fixes to C++ Support
 - Fix crash when parsing nested requirement. Fixes:
   (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
 
+- Fixed a crash caused by using return type requirement in a lambda. Fixes:
+  (`#64607 <https://github.com/llvm/llvm-project/issues/64607>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 37e5b9cad08bc9..bbaead33a6efe1 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1479,6 +1479,14 @@ namespace {
       return Result;
     }
 
+    StmtResult TransformLambdaBody(LambdaExpr *E, Stmt *Body) {
+      bool Prev = EvaluateConstraints;
+      EvaluateConstraints = true;
+      StmtResult Stmt = inherited::TransformLambdaBody(E, Body);
+      EvaluateConstraints = Prev;
+      return Stmt;
+    }
+
     ExprResult TransformRequiresExpr(RequiresExpr *E) {
       LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
       ExprResult TransReq = inherited::TransformRequiresExpr(E);
diff --git a/clang/test/SemaTemplate/concepts-lambda.cpp b/clang/test/SemaTemplate/concepts-lambda.cpp
index 8a184cbf4e9bc2..0a58137055a381 100644
--- a/clang/test/SemaTemplate/concepts-lambda.cpp
+++ b/clang/test/SemaTemplate/concepts-lambda.cpp
@@ -116,3 +116,36 @@ static_assert(E<int>);
 // expected-note@-11{{because 'Q.template operator()<float>()' would be invalid: no matching member function for call to 'operator()'}}
 }
 }
+
+namespace ReturnTypeConstraintInLambda {
+template <typename T>
+concept C1 = true;
+
+template <class T>
+concept test = [] {
+  return requires(T t) {
+    { t } -> C1;
+  };
+}();
+
+static_assert(test<int>);
+
+template <typename T>
+concept C2 = true;
+struct S1 {
+  int f1() { return 1; }
+};
+
+void foo() {
+  auto make_caller = []<auto member> {
+    return [](S1 *ps) {
+      if constexpr (requires {
+                      { (ps->*member)() } -> C2;
+                    })
+        ;
+    };
+  };
+
+  auto caller = make_caller.operator()<&S1::f1>();
+}
+} // namespace ReturnTypeConstraintInLambda

[erichkeane]

This is a not-awful way of solving this until we stop instantiating the body of the lambda. Could you please comment to that effect in TransformLambdaBody?

Also would like @cor3ntin to take a look as well, else i'm ok with this.

[erichkeane]

I'm still ok with it as-is, but would like @cor3ntin to take a run/do the approval.

[cor3ntin]

LGTM.
I do hope that @erichkeane will be able to work on delayed instantiation though!