[flang][Evaluate] Restrict ConstantBase constructor overload (#138456)

kparzysz · web-flow · commit 304c7a87d01b · 2025-05-06T12:00:29.000-05:00
ConstantBase has a constructor that takes a value of any type as an
input: template &lt;typename T&gt; ConstantBase(const T &amp;). A derived type
Constant&lt;T&gt; is a member of many Expr&lt;T&gt; classes (as an alternative in
the member variant).

When trying (erroneously) to create Expr&lt;T&gt; from a wrong input, if the
specific instance of Expr&lt;T&gt; contains Constant&lt;T&gt;, it's that constructor
that will be instantiated, leading to cryptic and confusing errors.

Eliminate the constructor from overload for invalid input values to help
produce more meaningful diagnostics.
diff --git a/flang/include/flang/Evaluate/constant.h b/flang/include/flang/Evaluate/constant.h
@@ -110,8 +110,12 @@ class ConstantBase : public ConstantBounds {
   using Result = RESULT;
   using Element = ELEMENT;
 
-  template <typename A>
+  // Constructor for creating ConstantBase from an actual value (i.e.
+  // literals, etc.)
+  template <typename A,
+      typename = std::enable_if_t<std::is_convertible_v<A, Element>>>
   ConstantBase(const A &x, Result res = Result{}) : result_{res}, values_{x} {}
+
   ConstantBase(ELEMENT &&x, Result res = Result{})
       : result_{res}, values_{std::move(x)} {}
   ConstantBase(
