[clang][Sema] Fix diagnostic for function overloading in extern "C"

clang/docs/ReleaseNotes.rst

@@ -261,6 +261,8 @@ Improvements to Clang's diagnostics
   compilation speed with modules. This warning is disabled by default and it needs
   to be explicitly enabled or by ``-Weverything``.
 
+- Improved diagnostic when trying to overload a function in an ``extern "C"`` context. (#GH80235)
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaDecl.cpp

@@ -3882,6 +3882,10 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
         if (New->isCXXClassMember() && New->isOutOfLine())
           Diag(New->getLocation(), diag::err_member_def_does_not_match_ret_type)
               << New << New->getReturnTypeSourceRange();
+        else if (Old->isExternC() && New->isExternC() &&
+                 !Old->hasAttr<OverloadableAttr>() &&
+                 !New->hasAttr<OverloadableAttr>())
+          Diag(New->getLocation(), diag::err_conflicting_types) << New;
         else
           Diag(New->getLocation(), diag::err_ovl_diff_return_type)
               << New->getReturnTypeSourceRange();

clang/test/SemaCXX/extern-c.cpp

@@ -77,6 +77,19 @@ namespace foo {
   }
 }
 
+namespace extern_ovl {
+  extern "C" {
+    __attribute__((overloadable))
+    void ovl_decl(void); // expected-note {{previous}}
+
+    __attribute__((overloadable))
+    int ovl_decl(int);
+
+    __attribute__((overloadable))
+    int ovl_decl(void); // expected-error {{functions that differ only in their return type}}
+  }
+}
+
 namespace linkage {
   namespace redecl {
     extern "C" {
@@ -88,6 +101,12 @@ namespace linkage {
       void linkage_redecl(double); // expected-error {{conflicting types}}
     }
   }
+  namespace redecl_2 {
+    extern "C" {
+      void linkage_redecl_2(); // expected-note {{previous}}
+      int linkage_redecl_2(int); // expected-error {{conflicting types}}
+    }
+  }
   namespace from_outer {
     void linkage_from_outer_1(); // expected-note {{previous}}
     void linkage_from_outer_2(); // expected-note {{previous}}