[clang]improve diagnosing redefined defaulted constructor with different exception specs (#69688)

clang/docs/ReleaseNotes.rst

@@ -348,6 +348,9 @@ Improvements to Clang's diagnostics
       |               ~~~~~~~~~^~~~~~~~
 - Clang now always diagnoses when using non-standard layout types in ``offsetof`` .
   (`#64619: <https://github.com/llvm/llvm-project/issues/64619>`_)
+- Clang now diagnoses redefined defaulted constructor when redefined
+  defaulted constructor with different exception specs.
+  (`#69094: <https://github.com/llvm/llvm-project/issues/69094>`_)
 - Clang now diagnoses use of variable-length arrays in C++ by default (and
   under ``-Wall`` in GNU++ mode). This is an extension supported by Clang and
   GCC, but is very easy to accidentally use without realizing it's a

clang/lib/Sema/SemaDecl.cpp

@@ -3922,18 +3922,6 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
   }
 
   if (getLangOpts().CPlusPlus) {
-    // C++1z [over.load]p2
-    //   Certain function declarations cannot be overloaded:
-    //     -- Function declarations that differ only in the return type,
-    //        the exception specification, or both cannot be overloaded.
-
-    // Check the exception specifications match. This may recompute the type of
-    // both Old and New if it resolved exception specifications, so grab the
-    // types again after this. Because this updates the type, we do this before
-    // any of the other checks below, which may update the "de facto" NewQType
-    // but do not necessarily update the type of New.
-    if (CheckEquivalentExceptionSpec(Old, New))
-      return true;
     OldQType = Context.getCanonicalType(Old->getType());
     NewQType = Context.getCanonicalType(New->getType());
 
@@ -4055,6 +4043,19 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
       }
     }
 
+    // C++1z [over.load]p2
+    //   Certain function declarations cannot be overloaded:
+    //     -- Function declarations that differ only in the return type,
+    //        the exception specification, or both cannot be overloaded.
+
+    // Check the exception specifications match. This may recompute the type of
+    // both Old and New if it resolved exception specifications, so grab the
+    // types again after this. Because this updates the type, we do this before
+    // any of the other checks below, which may update the "de facto" NewQType
+    // but do not necessarily update the type of New.
+    if (CheckEquivalentExceptionSpec(Old, New))
+      return true;
+
     // C++11 [dcl.attr.noreturn]p1:
     //   The first declaration of a function shall specify the noreturn
     //   attribute if any declaration of that function specifies the noreturn

clang/test/SemaCXX/diagnose-prioritiy-exception-redefining.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -fcxx-exceptions -std=c++11 %s
+
+struct ExplicitlySpecialMethod {
+  ExplicitlySpecialMethod() = default;
+};
+ExplicitlySpecialMethod::ExplicitlySpecialMethod() {} // expected-error{{definition of explicitly defaulted default constructor}}
+
+struct ImplicitlySpecialMethod {};
+ImplicitlySpecialMethod::ImplicitlySpecialMethod() {} // expected-error{{definition of implicitly declared default constructor}}