[clang] CTAD: Respect requires-clause of the original function template for the synthesized deduction guide (#84913)

We ignored the require-clause of the original template when building the deduction guide for type-alias CTAD, this resulted in accepting code which should be rejected (see the test case). This patch fixes it, part of #84492.

Author: hokein

clang/lib/Sema/SemaTemplate.cpp

@@ -2906,18 +2906,27 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
           Context.getCanonicalTemplateArgument(
               Context.getInjectedTemplateArg(NewParam));
     }
-    // FIXME: implement the associated constraint per C++
+    // Substitute new template parameters into requires-clause if present.
+    Expr *RequiresClause = nullptr;
+    if (Expr *InnerRC = F->getTemplateParameters()->getRequiresClause()) {
+      MultiLevelTemplateArgumentList Args;
+      Args.setKind(TemplateSubstitutionKind::Rewrite);
+      Args.addOuterTemplateArguments(TemplateArgsForBuildingFPrime);
+      ExprResult E = SemaRef.SubstExpr(InnerRC, Args);
+      if (E.isInvalid())
+        return;
+      RequiresClause = E.getAs<Expr>();
+    }
+    // FIXME: implement the is_deducible constraint per C++
     // [over.match.class.deduct]p3.3:
-    //    The associated constraints ([temp.constr.decl]) are the
-    //    conjunction of the associated constraints of g and a
-    //    constraint that is satisfied if and only if the arguments
+    //    ... and a constraint that is satisfied if and only if the arguments
     //    of A are deducible (see below) from the return type.
     auto *FPrimeTemplateParamList = TemplateParameterList::Create(
         Context, AliasTemplate->getTemplateParameters()->getTemplateLoc(),
         AliasTemplate->getTemplateParameters()->getLAngleLoc(),
         FPrimeTemplateParams,
         AliasTemplate->getTemplateParameters()->getRAngleLoc(),
-        /*RequiresClause=*/nullptr);
+        /*RequiresClause=*/RequiresClause);
 
     // To form a deduction guide f' from f, we leverage clang's instantiation
     // mechanism, we construct a template argument list where the template

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -230,3 +230,20 @@ using AFoo = Foo<U>*; // expected-note {{template is declared here}}
 
 AFoo s = {1}; // expected-error {{alias template 'AFoo' requires template arguments; argument deduction only allowed for}}
 } // namespace test17
+
+namespace test18 {
+template<typename T>
+concept False = false; // expected-note {{because 'false' evaluated to false}}
+
+template <typename T> struct Foo { T t; };
+
+template<typename T> requires False<T> // expected-note {{because 'int' does not satisfy 'False'}}
+Foo(T) -> Foo<int>;
+
+template <typename U>
+using Bar = Foo<U>; // expected-note {{could not match 'Foo<type-parameter-0-0>' against 'int'}} \
+                    // expected-note {{candidate template ignored: constraints not satisfied}} \
+                    // expected-note {{candidate function template not viable}}
+
+Bar s = {1}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments}}
+} // namespace test18