diff --git a/src/libs/3rdparty/cplusplus/FullySpecifiedType.h b/src/libs/3rdparty/cplusplus/FullySpecifiedType.h
index 498e190330e..4c7a21ddc35 100644
--- a/src/libs/3rdparty/cplusplus/FullySpecifiedType.h
+++ b/src/libs/3rdparty/cplusplus/FullySpecifiedType.h
@@ -33,7 +33,7 @@ public:
     ~FullySpecifiedType();
 
     bool isValid() const;
-    operator bool() const;
+    explicit operator bool() const;
 
     Type *type() const;
     void setType(Type *type);
diff --git a/src/libs/cplusplus/pp-engine.cpp b/src/libs/cplusplus/pp-engine.cpp
index d3b49953649..fa96c54176d 100644
--- a/src/libs/cplusplus/pp-engine.cpp
+++ b/src/libs/cplusplus/pp-engine.cpp
@@ -297,7 +297,7 @@ public:
         trivial.utf16charOffset = last->utf16charOffset;
     }
 
-    inline operator bool() const
+    inline explicit operator bool() const
     { return first != last; }
 
     inline bool isValid() const
diff --git a/src/libs/glsl/glslsemantic.h b/src/libs/glsl/glslsemantic.h
index 3c992514142..cb8dc464a73 100644
--- a/src/libs/glsl/glslsemantic.h
+++ b/src/libs/glsl/glslsemantic.h
@@ -56,7 +56,7 @@ public:
             return true;
         }
 
-        operator bool() const { return isValid(); }
+        explicit operator bool() const { return isValid(); }
 
         const Type *type;
         bool isConstant;
diff --git a/src/libs/qmljs/qmljsmodelmanagerinterface.h b/src/libs/qmljs/qmljsmodelmanagerinterface.h
index 18ed602fc40..551db94da55 100644
--- a/src/libs/qmljs/qmljsmodelmanagerinterface.h
+++ b/src/libs/qmljs/qmljsmodelmanagerinterface.h
@@ -78,7 +78,7 @@ public:
             , tryQmlDump(false), qmlDumpHasRelocatableFlag(true)
         { }
 
-        operator bool() const
+        explicit operator bool() const
         { return ! project.isNull(); }
 
         bool isValid() const
diff --git a/src/libs/qtcreatorcdbext/iinterfacepointer.h b/src/libs/qtcreatorcdbext/iinterfacepointer.h
index 4fbbad1590d..674e880a9ad 100644
--- a/src/libs/qtcreatorcdbext/iinterfacepointer.h
+++ b/src/libs/qtcreatorcdbext/iinterfacepointer.h
@@ -51,7 +51,7 @@ public:
     inline IInterface *operator->() const { return m_instance; }
     inline IInterface *data() const { return m_instance; }
     inline bool isNull() const   { return m_instance == 0; }
-    inline operator bool() const { return !isNull(); }
+    explicit inline operator bool() const { return !isNull(); }
     inline HRESULT hr() const { return m_hr; }
 
     // This is for creating a IDebugClient from scratch. Not matches by a constructor,
diff --git a/src/libs/qtcreatorcdbext/symbolgroupvalue.h b/src/libs/qtcreatorcdbext/symbolgroupvalue.h
index 9bbef1ff822..9a36d02e38a 100644
--- a/src/libs/qtcreatorcdbext/symbolgroupvalue.h
+++ b/src/libs/qtcreatorcdbext/symbolgroupvalue.h
@@ -63,7 +63,7 @@ public:
     explicit SymbolGroupValue(SymbolGroupNode *node, const SymbolGroupValueContext &c);
     SymbolGroupValue();
 
-    operator bool() const { return isValid(); }
+    explicit operator bool() const { return isValid(); }
     bool isValid() const;
 
     // Access children by name or index (0-based)
diff --git a/src/plugins/coreplugin/vcsmanager.cpp b/src/plugins/coreplugin/vcsmanager.cpp
index f4ec9e84735..1214215f242 100644
--- a/src/plugins/coreplugin/vcsmanager.cpp
+++ b/src/plugins/coreplugin/vcsmanager.cpp
@@ -511,7 +511,7 @@ public:
         ExtensionSystem::PluginManager::addObject(watch);
     }
 
-    operator bool() { return m_watched; }
+    explicit operator bool() { return m_watched; }
     bool operator !() { return !m_watched; }
     T &operator*() { return *m_watched; }
     T *operator->() { return m_watched; }
diff --git a/src/plugins/designer/editordata.h b/src/plugins/designer/editordata.h
index aada8d9b54e..cff993a3010 100644
--- a/src/plugins/designer/editordata.h
+++ b/src/plugins/designer/editordata.h
@@ -42,7 +42,7 @@ namespace Internal {
 class EditorData
 {
 public:
-    operator bool() const { return formWindowEditor != 0; }
+    explicit operator bool() const { return formWindowEditor != 0; }
 
     FormWindowEditor *formWindowEditor = nullptr;
     SharedTools::WidgetHost *widgetHost = nullptr;