[clang] Fix CTAD not work for function-type and array-type arguments.

[hokein]

Fixes #51710.

When transforming a constructor into a corresponding deduction guide, the decayed types (function/array type) were not handled properly which made clang fail to compile valid code. The patch teaches clang handle these decayed type in the transformation.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

Changes

Fixes #51710

---

Full diff: https://github.com/llvm/llvm-project/pull/78159.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+7-1)
• (added) clang/test/SemaCXX/ctad-decay.cpp (+26)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 4aba054e252af24..82302bae6d6dc28 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -726,6 +726,8 @@ Bug Fixes in This Version
 - Fix an issue where clang cannot find conversion function with template
   parameter when instantiation of template class.
   Fixes (`#77583 <https://github.com/llvm/llvm-project/issues/77583>`_)
+- Fix an issue where CTAD fails for function-type/array-type arguments.
+  Fixes (`#51710 <https://github.com/llvm/llvm-project/issues/51710>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 5fcc39ec7005228..3052b01f7f12ac3 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2587,12 +2587,18 @@ struct ConvertConstructorToDeductionGuideTransform {
                           : ParamTy->isRValueReferenceType() ? VK_XValue
                                                              : VK_PRValue);
     }
+    // Handle arrays and functions decay.
+    auto NewType = NewDI->getType();
+    if (NewType->isArrayType())
+      NewType = SemaRef.Context.getArrayDecayedType(NewType);
+    else if (NewType->isFunctionType())
+      NewType = SemaRef.Context.getPointerType(NewType);
 
     ParmVarDecl *NewParam = ParmVarDecl::Create(SemaRef.Context, DC,
                                                 OldParam->getInnerLocStart(),
                                                 OldParam->getLocation(),
                                                 OldParam->getIdentifier(),
-                                                NewDI->getType(),
+                                                NewType,
                                                 NewDI,
                                                 OldParam->getStorageClass(),
                                                 NewDefArg.get());
diff --git a/clang/test/SemaCXX/ctad-decay.cpp b/clang/test/SemaCXX/ctad-decay.cpp
new file mode 100644
index 000000000000000..97063e8fb75659f
--- /dev/null
+++ b/clang/test/SemaCXX/ctad-decay.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-unused-value -std=c++20 %s
+// expected-no-diagnostics
+
+namespace GH51710 {
+
+template<typename T>
+struct A{
+  A(T f());
+  A(int f(), T);
+
+  A(T array[10]);
+  A(int array[10], T);
+};
+
+int foo();
+
+void bar() {
+  A test1(foo);
+  A test2(foo, 1);
+
+  int array[10];
+  A test3(array);
+  A test4(array, 1);
+}
+
+} // namespace GH51710

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[cor3ntin]

can you move that test to clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp ?

[hokein]

Sure, done.

[cor3ntin]

Can you add tests for incomplete array (in another struct ie

template<typename T>
struct B {
   B(T array[]) {}
   B(int array[], T) {}
};

[hokein]

Done.

[cor3ntin]

I think we can just use SemaRef.getDecayedType(NewDI->getType()), it covers both case

[hokein]

Good point, thanks. I was not aware of this API.

[cor3ntin]

LGTM, thanks

[shafik]

Can you please provide a more detailed summary, since these are usually what goes in the git log. It should describe the cause and the approach of the fix. It is also helpful for reviewers as well.

[shafik]

LGTM after more detailed summary provided.

[hokein]

Can you please provide a more detailed summary, since these are usually what goes in the git log. It should describe the cause and the approach of the fix. It is also helpful for reviewers as well.

Done, added details in the description.

[shafik]

Can you please provide a more detailed summary, since these are usually what goes in the git log. It should describe the cause and the approach of the fix. It is also helpful for reviewers as well.

Done, added details in the description.

Thank you! Much appreciated :-)