Perform type-checking for a converted constant expression in a template

argument even if the expression is value-dependent (we need to suppress the
final portion of the narrowing check, but the rest of the checking can still be
done eagerly).

This affects template template argument validity and partial ordering under
p0522r0.

llvm-svn: 290276

clang/include/clang/Sema/Overload.h

@@ -121,7 +121,11 @@ namespace clang {
 
     /// A narrowing conversion, because a non-constant-expression variable might
     /// have got narrowed.
-    NK_Variable_Narrowing
+    NK_Variable_Narrowing,
+
+    /// Cannot tell whether this is a narrowing conversion because the
+    /// expression is value-dependent.
+    NK_Dependent_Narrowing,
   };
 
   /// StandardConversionSequence - represents a standard conversion

clang/lib/Sema/SemaInit.cpp

@@ -6816,7 +6816,7 @@ InitializationSequence::Perform(Sema &S,
       CurInit = CurInitExprRes;
 
       if (Step->Kind == SK_ConversionSequenceNoNarrowing &&
-          S.getLangOpts().CPlusPlus && !CurInit.get()->isValueDependent())
+          S.getLangOpts().CPlusPlus)
         DiagnoseNarrowingInInitList(S, *Step->ICS, SourceType, Entity.getType(),
                                     CurInit.get());
 
@@ -8070,6 +8070,7 @@ static void DiagnoseNarrowingInInitList(Sema &S,
   switch (SCS->getNarrowingKind(S.Context, PostInit, ConstantValue,
                                 ConstantType)) {
   case NK_Not_Narrowing:
+  case NK_Dependent_Narrowing:
     // No narrowing occurred.
     return;
 

clang/lib/Sema/SemaOverload.cpp

@@ -329,6 +329,11 @@ StandardConversionSequence::getNarrowingKind(ASTContext &Ctx,
     } else if (FromType->isIntegralType(Ctx) && ToType->isRealFloatingType()) {
       llvm::APSInt IntConstantValue;
       const Expr *Initializer = IgnoreNarrowingConversion(Converted);
+
+      // If it's value-dependent, we can't tell whether it's narrowing.
+      if (Initializer->isValueDependent())
+        return NK_Dependent_Narrowing;
+
       if (Initializer &&
           Initializer->isIntegerConstantExpr(IntConstantValue, Ctx)) {
         // Convert the integer to the floating type.
@@ -362,6 +367,11 @@ StandardConversionSequence::getNarrowingKind(ASTContext &Ctx,
         Ctx.getFloatingTypeOrder(FromType, ToType) == 1) {
       // FromType is larger than ToType.
       const Expr *Initializer = IgnoreNarrowingConversion(Converted);
+
+      // If it's value-dependent, we can't tell whether it's narrowing.
+      if (Initializer->isValueDependent())
+        return NK_Dependent_Narrowing;
+
       if (Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
         // Constant!
         assert(ConstantValue.isFloat());
@@ -403,6 +413,11 @@ StandardConversionSequence::getNarrowingKind(ASTContext &Ctx,
       // Not all values of FromType can be represented in ToType.
       llvm::APSInt InitializerValue;
       const Expr *Initializer = IgnoreNarrowingConversion(Converted);
+
+      // If it's value-dependent, we can't tell whether it's narrowing.
+      if (Initializer->isValueDependent())
+        return NK_Dependent_Narrowing;
+
       if (!Initializer->isIntegerConstantExpr(InitializerValue, Ctx)) {
         // Such conversions on variables are always narrowing.
         return NK_Variable_Narrowing;
@@ -5289,6 +5304,9 @@ static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
   QualType PreNarrowingType;
   switch (SCS->getNarrowingKind(S.Context, Result.get(), PreNarrowingValue,
                                 PreNarrowingType)) {
+  case NK_Dependent_Narrowing:
+    // Implicit conversion to a narrower type, but the expression is
+    // value-dependent so we can't tell whether it's actually narrowing.
   case NK_Variable_Narrowing:
     // Implicit conversion to a narrower type, and the value is not a constant
     // expression. We'll diagnose this in a moment.
@@ -5307,6 +5325,11 @@ static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
     break;
   }
 
+  if (Result.get()->isValueDependent()) {
+    Value = APValue();
+    return Result;
+  }
+
   // Check the expression is a constant expression.
   SmallVector<PartialDiagnosticAt, 8> Notes;
   Expr::EvalResult Eval;

clang/lib/Sema/SemaTemplate.cpp

@@ -5059,13 +5059,6 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
   }
 
   if (getLangOpts().CPlusPlus1z) {
-    // FIXME: We can do some limited checking for a value-dependent but not
-    // type-dependent argument.
-    if (Arg->isValueDependent()) {
-      Converted = TemplateArgument(Arg);
-      return Arg;
-    }
-
     // C++1z [temp.arg.nontype]p1:
     //   A template-argument for a non-type template parameter shall be
     //   a converted constant expression of the type of the template-parameter.
@@ -5075,6 +5068,13 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     if (ArgResult.isInvalid())
       return ExprError();
 
+    // For a value-dependent argument, CheckConvertedConstantExpression is
+    // permitted (and expected) to be unable to determine a value.
+    if (ArgResult.get()->isValueDependent()) {
+      Converted = TemplateArgument(Arg);
+      return Arg;
+    }
+
     QualType CanonParamType = Context.getCanonicalType(ParamType);
 
     // Convert the APValue to a TemplateArgument.

clang/test/SemaTemplate/temp_arg_nontype_cxx1z.cpp

@@ -122,13 +122,13 @@ namespace DeduceDifferentType {
   int a_exp = a<3>(A<3>());
 
   template<decltype(nullptr)> struct B {};
-  template<int *P> int b(B<P>); // expected-note {{does not have the same type}} expected-note {{not implicitly convertible}}
+  template<int *P> int b(B<P>); // expected-error {{value of type 'int *' is not implicitly convertible to 'decltype(nullptr)'}}
   int b_imp = b(B<nullptr>()); // expected-error {{no matching function}}
   int b_exp = b<nullptr>(B<nullptr>()); // expected-error {{no matching function}}
 
   struct X { constexpr operator int() { return 0; } } x;
   template<X &> struct C {};
-  template<int N> int c(C<N>); // expected-note {{does not have the same type}} expected-note {{not implicitly convertible}}
+  template<int N> int c(C<N>); // expected-error {{value of type 'int' is not implicitly convertible to 'DeduceDifferentType::X &'}}
   int c_imp = c(C<x>()); // expected-error {{no matching function}}
   int c_exp = c<x>(C<x>()); // expected-error {{no matching function}}