[clang] [clang] Find conversion function templates for in-class specializations (PR #218316)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Kunal Dubey  (xakep8)

<details>
<summary>Changes</summary>

Allow redeclaration lookup to consider conversion function templates allowing Clang to match an in-class specialization such as `template<> operator int()` against a prior conversion function template `template<class T> operator T()`.

Fixes #<!-- -->218261 

---
Full diff: https://github.com/llvm/llvm-project/pull/218316.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaLookup.cpp (+1-1) 
- (modified) clang/test/SemaCXX/conversion-function.cpp (+18) 


``````````diff
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 43129800e9813..b385392150d6c 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1158,7 +1158,7 @@ static bool LookupDirect(Sema &S, LookupResult &R, const DeclContext *DC) {
   //   name lookup. Instead, any conversion function templates visible in the
   //   context of the use are considered. [...]
   const CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
-  if (!Record->isCompleteDefinition())
+  if (!Record->isCompleteDefinition() && !R.isForRedeclaration())
     return Found;
 
   // For conversion operators, 'operator auto' should only match
diff --git a/clang/test/SemaCXX/conversion-function.cpp b/clang/test/SemaCXX/conversion-function.cpp
index 717c73c4786eb..e00553fbb20a3 100644
--- a/clang/test/SemaCXX/conversion-function.cpp
+++ b/clang/test/SemaCXX/conversion-function.cpp
@@ -473,6 +473,24 @@ struct S {
 };
 }
 
+#if __cplusplus >= 201103L
+namespace GH218261 {
+  struct S {
+    template <typename T>
+    constexpr operator T() const {
+      return 10;
+    }
+
+    template <>
+    constexpr operator int() const {
+      return 4;
+    }
+  };
+
+  static_assert(S().operator int() == 4, "");
+}
+#endif
+
 #if __cplusplus >= 201103L
 namespace dependent_conversion_function_id_lookup {
 namespace gh77583 {

``````````

</details>


https://github.com/llvm/llvm-project/pull/218316