[Clang] Correct the order of substituted arguments in CTAD alias guides

clang/docs/ReleaseNotes.rst

@@ -907,6 +907,7 @@ Bug Fixes to C++ Support
   (`LWG3929 <https://wg21.link/LWG3929>`__.) (#GH121278)
 - Clang now identifies unexpanded parameter packs within the type constraint on a non-type template parameter. (#GH88866)
 - Fixed an issue while resolving type of expression indexing into a pack of values of non-dependent type (#GH121242)
+- Fixed incorrect construction of template arguments for CTAD alias guides when type constraints are applied. (#GH122134)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -996,7 +996,7 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
       F->getTemplateParameters()->size());
 
   // FIXME: DeduceTemplateArguments stops immediately at the first
-  // non-deducible template argument. However, this doesn't seem to casue
+  // non-deducible template argument. However, this doesn't seem to cause
   // issues for practice cases, we probably need to extend it to continue
   // performing deduction for rest of arguments to align with the C++
   // standard.
@@ -1053,25 +1053,6 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
     TransformedDeducedAliasArgs[AliasTemplateParamIdx] = NewTemplateArgument;
   }
   unsigned FirstUndeducedParamIdx = FPrimeTemplateParams.size();
-  //   ...followed by the template parameters of f that were not deduced
-  //   (including their default template arguments)
-  for (unsigned FTemplateParamIdx : NonDeducedTemplateParamsInFIndex) {
-    auto *TP = F->getTemplateParameters()->getParam(FTemplateParamIdx);
-    MultiLevelTemplateArgumentList Args;
-    Args.setKind(TemplateSubstitutionKind::Rewrite);
-    // We take a shortcut here, it is ok to reuse the
-    // TemplateArgsForBuildingFPrime.
-    Args.addOuterTemplateArguments(TemplateArgsForBuildingFPrime);
-    NamedDecl *NewParam = transformTemplateParameter(
-        SemaRef, F->getDeclContext(), TP, Args, FPrimeTemplateParams.size(),
-        getDepthAndIndex(TP).first);
-    FPrimeTemplateParams.push_back(NewParam);
-
-    assert(TemplateArgsForBuildingFPrime[FTemplateParamIdx].isNull() &&
-           "The argument must be null before setting");
-    TemplateArgsForBuildingFPrime[FTemplateParamIdx] =
-        Context.getInjectedTemplateArg(NewParam);
-  }
 
   // To form a deduction guide f' from f, we leverage clang's instantiation
   // mechanism, we construct a template argument list where the template
@@ -1086,18 +1067,14 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
   //  2) non-deduced template parameters of f: rebuild a
   //  template argument;
   //
-  // 2) has been built already (when rebuilding the new template
-  // parameters), we now perform 1).
+  // We now perform 1), as case 2) might refer to substituted 1).
   MultiLevelTemplateArgumentList Args;
   Args.setKind(TemplateSubstitutionKind::Rewrite);
   Args.addOuterTemplateArguments(TransformedDeducedAliasArgs);
   for (unsigned Index = 0; Index < DeduceResults.size(); ++Index) {
     const auto &D = DeduceResults[Index];
     if (D.isNull()) {
-      // 2): Non-deduced template parameter has been built already.
-      assert(!TemplateArgsForBuildingFPrime[Index].isNull() &&
-             "template arguments for non-deduced template parameters should "
-             "be been set!");
+      // 2): Non-deduced template parameters would be substituted later.
       continue;
     }
     TemplateArgumentLoc Input =
@@ -1110,6 +1087,27 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
     }
   }
 
+  // Case 2)
+  //   ...followed by the template parameters of f that were not deduced
+  //   (including their default template arguments)
+  for (unsigned FTemplateParamIdx : NonDeducedTemplateParamsInFIndex) {
+    auto *TP = F->getTemplateParameters()->getParam(FTemplateParamIdx);
+    MultiLevelTemplateArgumentList Args;
+    Args.setKind(TemplateSubstitutionKind::Rewrite);
+    // We take a shortcut here, it is ok to reuse the
+    // TemplateArgsForBuildingFPrime.
+    Args.addOuterTemplateArguments(TemplateArgsForBuildingFPrime);
+    NamedDecl *NewParam = transformTemplateParameter(
+        SemaRef, F->getDeclContext(), TP, Args, FPrimeTemplateParams.size(),
+        getDepthAndIndex(TP).first);
+    FPrimeTemplateParams.push_back(NewParam);
+
+    assert(TemplateArgsForBuildingFPrime[FTemplateParamIdx].isNull() &&
+           "The argument must be null before setting");
+    TemplateArgsForBuildingFPrime[FTemplateParamIdx] =
+        Context.getInjectedTemplateArg(NewParam);
+  }
+
   auto *TemplateArgListForBuildingFPrime =
       TemplateArgumentList::CreateCopy(Context, TemplateArgsForBuildingFPrime);
   // Form the f' by substituting the template arguments into f.

clang/test/SemaTemplate/deduction-guide.cpp

@@ -478,3 +478,53 @@ A a{.f1 = {1}};
 // CHECK-NEXT:     `-DeclRefExpr {{.+}} <col:10> 'int' NonTypeTemplateParm {{.+}} 'N' 'int'
 
 } // namespace GH83368
+
+namespace GH122134 {
+
+template <class, class>
+concept Constraint = true;
+
+template <class T, int> struct Struct {
+  Struct(Constraint<T> auto) {}
+};
+
+template <int N = 0> using Test = Struct<int, N>;
+
+Test test(42);
+
+// CHECK-LABEL: Dumping GH122134::<deduction guide for Test>:
+// CHECK-NEXT: FunctionTemplateDecl {{.*}} implicit <deduction guide for Test>
+// CHECK-NEXT: |-NonTypeTemplateParmDecl {{.*}} 'int' depth 0 index 0 N
+// CHECK-NEXT: | `-TemplateArgument {{.*}} expr '0'
+// CHECK-NEXT: |   `-IntegerLiteral {{.*}} 'int' 0
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.*}} Concept {{.*}} 'Constraint' depth 0 index 1 auto:1
+// CHECK-NEXT: | `-ConceptSpecializationExpr {{.*}} 'bool' Concept {{.*}} 'Constraint'
+// CHECK-NEXT: |   |-ImplicitConceptSpecializationDecl {{.*}}
+// CHECK-NEXT: |   | |-TemplateArgument type 'type-parameter-0-1'
+// CHECK-NEXT: |   | | `-TemplateTypeParmType {{.*}} 'type-parameter-0-1' dependent depth 0 index 1
+// CHECK-NEXT: |   | `-TemplateArgument type 'int'
+// CHECK-NEXT: |   |   `-BuiltinType {{.*}} 'int'
+// CHECK-NEXT: |   |-TemplateArgument {{.*}} type 'auto:1':'type-parameter-0-1'
+// CHECK-NEXT: |   | `-TemplateTypeParmType {{.*}} 'auto:1' dependent depth 0 index 1
+// CHECK-NEXT: |   |   `-TemplateTypeParm {{.*}} 'auto:1'
+// CHECK-NEXT: |   `-TemplateArgument {{.*}} type 'int'
+// CHECK-NEXT: |     `-BuiltinType {{.*}} 'int'
+// CHECK-NEXT: |-TypeTraitExpr {{.*}} 'bool' __is_deducible
+// CHECK-NEXT: | |-DeducedTemplateSpecializationType {{.*}} 'GH122134::Test' dependent
+// CHECK-NEXT: | | `-name: 'GH122134::Test'
+// CHECK-NEXT: | |   `-TypeAliasTemplateDecl {{.*}} Test
+// CHECK-NEXT: | `-TemplateSpecializationType {{.*}} 'Struct<int, N>' dependent
+// CHECK-NEXT: |   |-name: 'Struct':'GH122134::Struct' qualified
+// CHECK-NEXT: |   | `-ClassTemplateDecl {{.*}} Struct
+// CHECK-NEXT: |   |-TemplateArgument type 'int'
+// CHECK-NEXT: |   | `-SubstTemplateTypeParmType {{.*}} 'int' sugar class depth 0 index 0 T
+// CHECK-NEXT: |   |   |-FunctionTemplate {{.*}} '<deduction guide for Struct>'
+// CHECK-NEXT: |   |   `-BuiltinType {{.*}} 'int'
+// CHECK-NEXT: |   `-TemplateArgument expr 'N'
+// CHECK-NEXT: |     `-SubstNonTypeTemplateParmExpr {{.*}} 'int'
+// CHECK-NEXT: |       |-NonTypeTemplateParmDecl {{.*}} 'int' depth 0 index 1
+// CHECK-NEXT: |       `-DeclRefExpr {{.*}} 'int' NonTypeTemplateParm {{.*}} 'N' 'int'
+// CHECK-NEXT: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for Test> 'auto (auto:1) -> Struct<int, N>'
+// CHECK-NEXT: | `-ParmVarDecl {{.*}} 'auto:1'
+
+} // namespace GH122134