[Clang] Defer the instantiation of explicit-specifier until constraint checking completes

clang/docs/ReleaseNotes.rst

@@ -682,6 +682,10 @@ Bug Fixes to C++ Support
   Fixes:
   (`#68769 <https://github.com/llvm/llvm-project/issues/68769>`_)
 
+- Clang now defers the instantiation of explicit specifier until constraint checking
+  completes (except deduction guides). Fixes:
+  (`#59827 <https://github.com/llvm/llvm-project/issues/59827>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/include/clang/Sema/Sema.h

@@ -10430,6 +10430,9 @@ class Sema final {
                                   const CXXConstructorDecl *Tmpl,
                             const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  ExplicitSpecifier instantiateExplicitSpecifier(
+      const MultiLevelTemplateArgumentList &TemplateArgs, ExplicitSpecifier ES);
+
   NamedDecl *FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
                           const MultiLevelTemplateArgumentList &TemplateArgs,
                           bool FindingInstantiatedContext = false);

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -3553,6 +3553,48 @@ static unsigned getPackIndexForParam(Sema &S,
   llvm_unreachable("parameter index would not be produced from template");
 }
 
+// if `Specialization` is a `CXXConstructorDecl` or `CXXConversionDecl`,
+// we'll try to instantiate and update its explicit specifier after constraint
+// checking.
+static Sema::TemplateDeductionResult instantiateExplicitSpecifierDeferred(
+    Sema &S, FunctionDecl *Specialization,
+    const MultiLevelTemplateArgumentList &SubstArgs,
+    TemplateDeductionInfo &Info, FunctionTemplateDecl *FunctionTemplate,
+    ArrayRef<TemplateArgument> DeducedArgs) {
+  auto GetExplicitSpecifier = [](FunctionDecl *D) {
+    return isa<CXXConstructorDecl>(D)
+               ? cast<CXXConstructorDecl>(D)->getExplicitSpecifier()
+               : cast<CXXConversionDecl>(D)->getExplicitSpecifier();
+  };
+  auto SetExplicitSpecifier = [](FunctionDecl *D, ExplicitSpecifier ES) {
+    isa<CXXConstructorDecl>(D)
+        ? cast<CXXConstructorDecl>(D)->setExplicitSpecifier(ES)
+        : cast<CXXConversionDecl>(D)->setExplicitSpecifier(ES);
+  };
+
+  ExplicitSpecifier ES = GetExplicitSpecifier(Specialization);
+  Expr *ExplicitExpr = ES.getExpr();
+  if (!ExplicitExpr)
+    return Sema::TDK_Success;
+  if (!ExplicitExpr->isValueDependent())
+    return Sema::TDK_Success;
+
+  Sema::InstantiatingTemplate Inst(
+      S, Info.getLocation(), FunctionTemplate, DeducedArgs,
+      Sema::CodeSynthesisContext::DeducedTemplateArgumentSubstitution, Info);
+  if (Inst.isInvalid())
+    return Sema::TDK_InstantiationDepth;
+  Sema::SFINAETrap Trap(S);
+  const ExplicitSpecifier InstantiatedES =
+      S.instantiateExplicitSpecifier(SubstArgs, ES);
+  if (InstantiatedES.isInvalid() || Trap.hasErrorOccurred()) {
+    Specialization->setInvalidDecl(true);
+    return Sema::TDK_SubstitutionFailure;
+  }
+  SetExplicitSpecifier(Specialization, InstantiatedES);
+  return Sema::TDK_Success;
+}
+
 /// Finish template argument deduction for a function template,
 /// checking the deduced template arguments for completeness and forming
 /// the function template specialization.
@@ -3682,6 +3724,17 @@ Sema::TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
     }
   }
 
+  // We skipped the instantiation of the explicit-specifier during the
+  // substitution of `FD` before. So, we try to instantiate it back if
+  // `Specialization` is either a constructor or a conversion function.
+  if (isa<CXXConstructorDecl, CXXConversionDecl>(Specialization)) {
+    if (TDK_Success != instantiateExplicitSpecifierDeferred(
+                           *this, Specialization, SubstArgs, Info,
+                           FunctionTemplate, DeducedArgs)) {
+      return TDK_SubstitutionFailure;
+    }
+  }
+
   if (OriginalCallArgs) {
     // C++ [temp.deduct.call]p4:
     //   In general, the deduction process attempts to find template argument

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -563,26 +563,24 @@ static void instantiateDependentAMDGPUFlatWorkGroupSizeAttr(
   S.addAMDGPUFlatWorkGroupSizeAttr(New, Attr, MinExpr, MaxExpr);
 }
 
-static ExplicitSpecifier
-instantiateExplicitSpecifier(Sema &S,
-                             const MultiLevelTemplateArgumentList &TemplateArgs,
-                             ExplicitSpecifier ES, FunctionDecl *New) {
+ExplicitSpecifier Sema::instantiateExplicitSpecifier(
+    const MultiLevelTemplateArgumentList &TemplateArgs, ExplicitSpecifier ES) {
   if (!ES.getExpr())
     return ES;
   Expr *OldCond = ES.getExpr();
   Expr *Cond = nullptr;
   {
     EnterExpressionEvaluationContext Unevaluated(
-        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
-    ExprResult SubstResult = S.SubstExpr(OldCond, TemplateArgs);
+        *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    ExprResult SubstResult = SubstExpr(OldCond, TemplateArgs);
     if (SubstResult.isInvalid()) {
       return ExplicitSpecifier::Invalid();
     }
     Cond = SubstResult.get();
   }
   ExplicitSpecifier Result(Cond, ES.getKind());
   if (!Cond->isTypeDependent())
-    S.tryResolveExplicitSpecifier(Result);
+    tryResolveExplicitSpecifier(Result);
   return Result;
 }
 
@@ -2073,8 +2071,8 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
 
   ExplicitSpecifier InstantiatedExplicitSpecifier;
   if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
-    InstantiatedExplicitSpecifier = instantiateExplicitSpecifier(
-        SemaRef, TemplateArgs, DGuide->getExplicitSpecifier(), DGuide);
+    InstantiatedExplicitSpecifier = SemaRef.instantiateExplicitSpecifier(
+        TemplateArgs, DGuide->getExplicitSpecifier());
     if (InstantiatedExplicitSpecifier.isInvalid())
       return nullptr;
   }
@@ -2453,11 +2451,25 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
     }
   }
 
-  ExplicitSpecifier InstantiatedExplicitSpecifier =
-      instantiateExplicitSpecifier(SemaRef, TemplateArgs,
-                                   ExplicitSpecifier::getFromDecl(D), D);
-  if (InstantiatedExplicitSpecifier.isInvalid())
-    return nullptr;
+  auto InstantiatedExplicitSpecifier = ExplicitSpecifier::getFromDecl(D);
+  // deduction guides need this
+  const bool CouldInstantiate =
+      InstantiatedExplicitSpecifier.getExpr() == nullptr ||
+      !InstantiatedExplicitSpecifier.getExpr()->isValueDependent();
+
+  // Delay the instantiation of the explicit-specifier until after the
+  // constraints are checked during template argument deduction.
+  if (CouldInstantiate ||
+      SemaRef.CodeSynthesisContexts.back().Kind !=
+          Sema::CodeSynthesisContext::DeducedTemplateArgumentSubstitution) {
+    InstantiatedExplicitSpecifier = SemaRef.instantiateExplicitSpecifier(
+        TemplateArgs, InstantiatedExplicitSpecifier);
+
+    if (InstantiatedExplicitSpecifier.isInvalid())
+      return nullptr;
+  } else {
+    InstantiatedExplicitSpecifier.setKind(ExplicitSpecKind::Unresolved);
+  }
 
   // Implicit destructors/constructors created for local classes in
   // DeclareImplicit* (see SemaDeclCXX.cpp) might not have an associated TSI.

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

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2a %s
+
+template <typename T1, typename T2> struct is_same {
+  static constexpr bool value = false;
+};
+
+template <typename T> struct is_same<T, T> {
+  static constexpr bool value = true;
+};
+
+template <class T, class U>
+concept SameHelper = is_same<T, U>::value;
+template <class T, class U>
+concept same_as = SameHelper<T, U> && SameHelper<U, T>;
+
+namespace deferred_instantiation {
+template <class X> constexpr X do_not_instantiate() { return nullptr; }
+
+struct T {
+  template <same_as<float> X> explicit(do_not_instantiate<X>()) T(X) {}
+
+  T(int) {}
+};
+
+T t(5);
+// expected-error@17{{cannot initialize}}
+// expected-note@20{{in instantiation of function template specialization}}
+// expected-note@30{{while substituting deduced template arguments}}
+// expected-note@30{{in instantiation of function template specialization}}
+T t2(5.0f);
+} // namespace deferred_instantiation