diff --git a/plugins/clang/codegen/adaptsignatureassistant.cpp b/plugins/clang/codegen/adaptsignatureassistant.cpp
--- a/plugins/clang/codegen/adaptsignatureassistant.cpp
+++ b/plugins/clang/codegen/adaptsignatureassistant.cpp
@@ -48,8 +48,19 @@
 
 bool isConstructor(const Declaration *functionDecl)
 {
+    ENSURE_CHAIN_READ_LOCKED
     auto classFun = dynamic_cast<const ClassFunctionDeclaration*>(functionDecl);
-    return classFun && classFun->isConstructor();
+    if(classFun) {
+        return classFun->isConstructor();
+    }
+
+    auto funDef = dynamic_cast<const FunctionDefinition*>(functionDecl);
+    if(funDef && funDef->hasDeclaration()) {
+        auto funDec = funDef->declaration();
+        auto classFun = dynamic_cast<const ClassFunctionDeclaration*>(funDec);
+        return classFun && classFun->isConstructor();
+    }
+    return false;
 }
 
 Signature getDeclarationSignature(const Declaration *functionDecl, const DUContext *functionCtxt, bool includeDefaults)
diff --git a/plugins/clang/tests/test_assistants.cpp b/plugins/clang/tests/test_assistants.cpp
--- a/plugins/clang/tests/test_assistants.cpp
+++ b/plugins/clang/tests/test_assistants.cpp
@@ -484,6 +484,14 @@
         << "class Foo { Foo(char c); };"
         << "Foo::Foo(char c)\n{}";
 
+    // see https://bugs.kde.org/show_bug.cgi?id=365420
+    QTest::newRow("no_retval_on_ctor_while_editing_definition")
+        << "class Foo {\nFoo(int a); \n};"
+        << "Foo::Foo(int a)\n{}"
+        << (QList<StateChange>() << StateChange(Testbed::CppDoc, Range(0,13,0,14), QStringLiteral("b"), SHOULD_ASSIST))
+        << "class Foo {\nFoo(int b); \n};"
+        << "Foo::Foo(int b)\n{}";
+
     // see https://bugs.kde.org/show_bug.cgi?id=298511
     QTest::newRow("change_return_type_header")
         << "struct Foo { int bar(); };"