[clang][Sema] Fix Issue: #166957 - type inconsistency for constrained auto variables

[KaushalMorankar]

Fixes #166957

This PR fixes an inconsistency where `getTypeSourceInfo()->getType()` and `getType()` return different `AutoType` instances with different `isDependentType()` values for constrained auto variables in dependent contexts.

Solution

The fix adds logic in ActOnVariableDeclarator (clang/lib/Sema/SemaDecl.cpp:7700-7719) to check if a constrained AutoType is being created, and ensures its dependency status matches the declaration context:
- In dependent contexts (e.g., templates): undeduced constrained autos are marked as dependent
- In non-dependent contexts: they are not marked as dependent

The fix reconstructs the AutoType with the correct dependency flag and updates the TypeSourceInfo to maintain consistency between both type retrieval methods.

Testing
Added comprehensive test coverage in clang/test/SemaCXX/constrained-auto-type-consistency.cpp which includes:
- Constrained auto in abbreviated function templates (dependent context)
- Constrained auto at namespace scope (non-dependent context) etc.

All tests pass with expected-no-diagnostics, confirming the fix resolves the inconsistency without introducing new issues.

Files Changed
- clang/lib/Sema/SemaDecl.cpp: Added logic to ensure AutoType dependency consistency
- clang/test/SemaCXX/constrained-auto-type-consistency.cpp: New test file (92 lines)

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Kaushal1042 (KaushalMorankar)

Changes

Fixes #<!-- -->166957

This PR fixes an inconsistency where `getTypeSourceInfo()-&gt;getType()` and `getType()` return different `AutoType` instances with different `isDependentType()` values for constrained auto variables in dependent contexts.

Solution

The fix adds logic in ActOnVariableDeclarator (clang/lib/Sema/SemaDecl.cpp:7700-7719) to check if a constrained AutoType is being created, and ensures its dependency status matches the declaration context:
- In dependent contexts (e.g., templates): undeduced constrained autos are marked as dependent
- In non-dependent contexts: they are not marked as dependent

The fix reconstructs the AutoType with the correct dependency flag and updates the TypeSourceInfo to maintain consistency between both type retrieval methods.

Testing
Added comprehensive test coverage in clang/test/SemaCXX/constrained-auto-type-consistency.cpp which includes:
- Constrained auto in abbreviated function templates (dependent context)
- Constrained auto at namespace scope (non-dependent context) etc.

All tests pass with expected-no-diagnostics, confirming the fix resolves the inconsistency without introducing new issues.

Files Changed
- clang/lib/Sema/SemaDecl.cpp: Added logic to ensure AutoType dependency consistency
- clang/test/SemaCXX/constrained-auto-type-consistency.cpp: New test file (92 lines)


---
Full diff: https://github.com/llvm/llvm-project/pull/167100.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaDecl.cpp (+20) 
- (added) clang/test/SemaCXX/constrained-auto-type-consistency.cpp (+92) 


``````````diff
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 086dd8ba1c670..99877615c50bf 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -7697,6 +7697,26 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     LookupResult &Previous, MultiTemplateParamsArg TemplateParamLists,
     bool &AddToScope, ArrayRef<BindingDecl *> Bindings) {
   QualType R = TInfo->getType();
+  
+  if (const AutoType *AT = R->getContainedAutoType()) {
+    if (AT->isConstrained()) {
+      bool IsInDependentContext = DC->isDependentContext();
+      bool ShouldBeDependent = IsInDependentContext && !AT->isDeduced();
+
+      if (ShouldBeDependent != AT->isDependentType()) {
+        QualType CanonAuto = Context.getAutoType(
+            AT->isDeduced() ? AT->getDeducedType() : QualType(),
+            AT->getKeyword(),
+            ShouldBeDependent,
+            false,
+            AT->getTypeConstraintConcept(),
+            AT->getTypeConstraintArguments());
+
+        R = Context.getQualifiedType(CanonAuto, R.getQualifiers());
+        TInfo = Context.getTrivialTypeSourceInfo(R, D.getBeginLoc());
+      }
+    }
+  }
   DeclarationName Name = GetNameForDeclarator(D).getName();
 
   IdentifierInfo *II = Name.getAsIdentifierInfo();
diff --git a/clang/test/SemaCXX/constrained-auto-type-consistency.cpp b/clang/test/SemaCXX/constrained-auto-type-consistency.cpp
new file mode 100644
index 0000000000000..7f6cec1604151
--- /dev/null
+++ b/clang/test/SemaCXX/constrained-auto-type-consistency.cpp
@@ -0,0 +1,92 @@
+// expected-no-diagnostics
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+namespace std {
+  template<typename T>
+  concept integral = __is_integral(T);
+
+  template<typename T>
+  concept floating_point = __is_floating_point(T);
+}
+
+// Constrained auto in abbreviated function template
+void find(auto value) {
+  std::integral auto var = value;
+}
+
+// Constrained auto at namespace scope (non-dependent context)
+// Should be deduced immediately
+std::integral auto globalVar = 42;
+
+// Multiple constrained autos in template function
+template<typename T>
+void multipleConstrainedAutos(T value) {
+  std::integral auto x = 10;
+  std::floating_point auto y = 3.14;
+  std::integral auto z = value; // dependent on T
+}
+
+// Constrained auto with qualifiers
+void testQualifiers(auto value) {
+  const std::integral auto cv1 = value;
+  std::integral auto const cv2 = value;
+}
+
+// Nested constrained auto
+void testNested(auto outer) {
+  auto lambda = [](auto inner) {
+    std::integral auto nested = inner;
+    return nested;
+  };
+
+  std::integral auto result = lambda(outer);
+}
+
+// Constrained auto with references
+void testReferences(auto value) {
+  std::integral auto& ref = value;
+  const std::integral auto& cref = value;
+}
+
+// Regular unconstrained auto (should not be affected by the fix)
+void testUnconstrainedAuto(auto value) {
+  auto regular = value;
+  decltype(auto) decl_auto = (value);
+}
+
+// Constrained auto in class template member
+template<typename T>
+struct Container {
+  void process(auto item) {
+    std::integral auto local = item;
+  }
+};
+
+// Constrained auto deduction from function call
+std::integral auto getInteger() { return 42; }
+
+void testFunctionReturn(auto param) {
+  std::integral auto fromFunc = getInteger();
+  std::integral auto fromParam = param;
+}
+
+// Ensure the fix doesn't break normal non-template constrained auto
+void normalFunction() {
+  std::integral auto x = 100;
+  // This should be immediately deduced to int, not dependent
+}
+
+// Instantiate templates to verify no crashes
+void instantiateAll() {
+  find(42);
+  multipleConstrainedAutos(5);
+  testQualifiers(7);
+  testNested(8);
+  int val = 10;
+  testReferences(val);
+  testUnconstrainedAuto(11);
+  Container<int> c;
+  c.process(12);
+  testFunctionReturn(13);
+  normalFunction();
+}

[zwuis]

Can you explain how the added test verifies that the issue is fixed?

[KaushalMorankar]

The bug was in the internal type consistency between getTypeSourceInfo()->getType() and getType(). I added compilation tests that serve as regression tests to ensure these patterns continue to compile correctly. While these tests don't directly verify the AST
consistency, they cover various scenarios where constrained auto is used, and I manually verified the fix by running AST dump on the test cases to confirm the AutoType objects have correct dependency flags and are consistent.

[zyn0217]

Did you AI-generate the patch? I would like to see some explanation as to why there was a discrepancy with TSI, and unfortunately your commit message doesn't seem to explain it very clearly.

If we were creating two AutoTypes, I think a right fix would be to combine them together in some way, other than adding odd checking in Sema without a reasonable explanation.

Besides, do we have similar issue on type constraints? E.g.

void foo(Concept auto Param) {}
Concept auto bar(int) {}

[shafik]

This looks like it came from this bug report: #166957

I also don't understand the solution and it is not clear to me this is correct.

[mizvekov]

I am abroad and can't review in depth right now.

But this inconsistency between the type and type source info is intentional and long standing.

I don't think there is a bug here except the interface is more complicated than the ideal.

The type source info holds the type as written, and the type holds the semantic type.

It would be nice if this merged the two at no extra cost, but I'd have to double check this later.

[erichkeane]

Doing this as a fixup seems wrong to me here. IF we wanted to merge these, we should do so at the root.

Also, your tests dont' really seem related to this, what is going on?