Library: prevent implicit bool conversion on iterator-like objects

...it should have been this way all the time.
Generic code might otherwise be ill guided to assume a conversion
from the Iterator to its value type, while in fact an explicit dereferentiation is necessary

src/gui/model/expander-revealer.hpp

@@ -107,6 +107,7 @@ namespace model {
              and bool{changeState_};
         }
       
+      explicit
       operator bool()  const
         {
           REQUIRE (canExpand());

src/lib/iter-adapter-ptr-deref.hpp

@@ -141,7 +141,11 @@ namespace lib {
           return *this;
         }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       
@@ -266,7 +270,11 @@ namespace lib {
           takeAddress();
         }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       

src/lib/iter-adapter-stl.hpp

@@ -72,11 +72,15 @@ namespace iter_stl {
       DistinctIter()            : i_(), prev_() { }
       DistinctIter(IT const& i) : i_(i),prev_() { memorise(); }
       
-      pointer   operator->() const { return i_; }
-      reference operator*()  const { return *i_;}
-      bool      isValid()    const { return i_; }
+      pointer   operator->()  const { return i_;  }
+      reference operator*()   const { return *i_; }
+      bool      isValid()     const { return i_;  }
       
-      operator bool()        const { return i_; }
+      explicit
+      operator bool() const
+        {
+          return bool{i_};
+        }
       
       
       DistinctIter&
@@ -472,8 +476,17 @@ namespace iter_stl {
       IterSnapshot& operator= (IterSnapshot const&) = default;
       IterSnapshot& operator= (IterSnapshot &&)     = default;
       
-      operator bool() const { return isValid(); }
-      size_t size()   const { return buffer_.size(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
+      
+      size_t
+      size()  const
+        {
+          return buffer_.size();
+        }
       
       
       

src/lib/iter-adapter.hpp

@@ -194,7 +194,11 @@ namespace lib {
         , pos_()
         { }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       /* === lumiera forward iterator concept === */
@@ -352,7 +356,11 @@ namespace lib {
         : core_()
         { }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       /* === lumiera forward iterator concept === */
@@ -491,7 +499,11 @@ namespace lib {
         , e_(oIter.getEnd())
         { }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       /* === lumiera forward iterator concept === */
@@ -599,7 +611,11 @@ namespace lib {
       
       // standard copy operations acceptable
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       
@@ -732,7 +748,11 @@ namespace lib {
         : i_(srcIter)
         { }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       

src/lib/itertools.hpp

@@ -205,7 +205,11 @@ namespace lib {
           hasData();
         }
       
-      operator bool() const { return isValid(); }
+      explicit
+      operator bool() const
+        {
+          return isValid();
+        }
       
       
       

src/lib/sync.hpp

@@ -257,7 +257,7 @@ namespace lib {
             return *this;
           }
         
-        operator bool() { return 0 != tv_sec; } // allows if (timeout_)....
+        explicit operator bool() { return 0 != tv_sec; } // allows if (timeout_)....
       };
     
     
@@ -368,7 +368,7 @@ namespace lib {
           }
         
         void setTimeout(ulong relative) {timeout_.setOffset(relative);}
-        bool isTimedWait()              {return (timeout_);}
+        bool isTimedWait()              {return bool{timeout_};}
       };
     
     typedef Mutex<Wrapped_ExclusiveMutex> NonrecursiveLock_NoWait;

tests/library/iter-tree-explorer-test.cpp

@@ -1188,7 +1188,7 @@ namespace test{
               bool
               checkPoint()  const
                 {
-                  return src;
+                  return bool{src};
                 }
               
               State&