[Clang] Set the decl context to the instantiated constructor when instantiating the explicit specifier

clang/docs/ReleaseNotes.rst

@@ -848,6 +848,9 @@ Bug Fixes to C++ Support
   completes (except deduction guides). Fixes:
   (`#59827 <https://github.com/llvm/llvm-project/issues/59827>`_)
 
+- Fixed the handling of concepts with lambda expression constraints in explicit specifiers.
+  Fixes: (`#67058 <https://github.com/llvm/llvm-project/issues/67058>`_)
+
 - Fix crash when parsing nested requirement. Fixes:
   (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
 

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -3585,6 +3585,7 @@ static Sema::TemplateDeductionResult instantiateExplicitSpecifierDeferred(
   if (Inst.isInvalid())
     return Sema::TDK_InstantiationDepth;
   Sema::SFINAETrap Trap(S);
+  Sema::ContextRAII InstantiatedContext(S, Specialization);
   const ExplicitSpecifier InstantiatedES =
       S.instantiateExplicitSpecifier(SubstArgs, ES);
   if (InstantiatedES.isInvalid() || Trap.hasErrorOccurred()) {

clang/test/SemaCXX/cxx2a-explicit-bool.cpp

@@ -743,3 +743,21 @@ struct S {
   explicit(1L) S(char, char, char);
 };
 } // namespace P1401
+
+#if __cplusplus > 201703L
+namespace GH67058 {
+template <class T>
+concept Q = requires(T t) { [](int *) {}(t); };
+struct A {
+  template <class T> explicit(Q<T>) A(T);
+};
+A a = 1;
+
+struct B { // expected-note+ {{candidate constructor}}
+  template <class T>
+  explicit(requires(T t) { [](int *) {}(t); })
+      B(T); // expected-note {{explicit constructor is not a candidate}}
+};
+B b = new int; // expected-error {{no viable conversion}}
+} // namespace GH67058
+#endif