Implement value type propagation on expansion

Currently, we only propagate types of sole, unquoted expansions (variable,
function call, or eval context), similar to NULL. To untypify the value,
simply quote it.

1 files changed, 36 insertions, 15 deletions

diff --git a/build2/parser b/build2/parser
index 9b326b1..e1f6e77 100644
--- a/build2/parser
+++ b/build2/parser
@@ -110,8 +110,9 @@ namespace build2
                             value&& rhs,
                             token_type kind);
 
-    // Return the value as well as the indication of whether this is a non-
-    // empty eval context (i.e., '()' potentially with whitespace in between).
+    // Return the value (can be NULL/typed) as well as the indication of
+    // whether this is a non-empty eval context (i.e., '()' potentially with
+    // whitespace in between).
     //
     pair<value, bool>
     parse_eval (token&, token_type&);
@@ -187,26 +188,46 @@ namespace build2
       return ns;
     }
 
-    // As above but return the result as a value (always untyped) indicating
-    // if it is NULL. The only way for the value to be NULL is if it is the
-    // result of a sole, unquoted variable expansion, function call, or
-    // context evaluation.
+    // As above but return the result as a value, which can be typed and NULL.
     //
     value
-    parse_names_value (token& t, token_type& tt,
-                       const char* what = "name",
-                       const string* separators = &name_separators)
+    parse_value (token& t, token_type& tt,
+                 const char* what = "name",
+                 const string* separators = &name_separators)
     {
       names ns;
-      return parse_names (
-        t, tt, ns, false, what, separators, 0, nullptr, nullptr, nullptr)
-        ? value (move (ns))
-        : value (nullptr);
+      pair<bool, const value_type*> p (
+        parse_names (
+          t, tt, ns, false, what, separators, 0, nullptr, nullptr, nullptr));
+
+      value r (p.second); // Potentially typed NULL value.
+
+      // This should not fail since we are typing the result of reversal from
+      // the typed value.
+      //
+      if (p.first) // Not NULL.
+        r.assign (move (ns), nullptr);
+
+      return r;
     }
 
-    // Append names and return true if the parsed value is NOT NULL.
+    // Append names and return the indication if the parsed value is NOT NULL
+    // (first) and whether it is typed (second).
     //
-    bool
+    // You may have noticed that what we return here is essentially a value
+    // and doing it this way (i.e., reversing it to untyped names and
+    // returning its type so that it can potentially be "typed back") is kind
+    // of backwards. The reason we are doing it this way is because in many
+    // places we expect things untyped and if we were to always return a
+    // (potentially typed) value, then we would have to reverse it in all
+    // those places. Still it may make sense to look into redesigning the
+    // whole thing one day.
+    //
+    // Currently the only way for the result to be NULL or have a type is if
+    // it is the result of a sole, unquoted variable expansion, function call,
+    // or context evaluation.
+    //
+    pair<bool, const value_type*>
     parse_names (token&, token_type&,
                  names&,
                  bool chunk = false,