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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-flang-semantics

Author: Krzysztof Parzyszek (kparzysz)

<details>
<summary>Changes</summary>

ConstantBase has a constructor that takes a value of any type as an input: template <typename T> ConstantBase(const T &). A derived type Constant<T> is a member of many Expr<T> classes (as an alternative in the member variant).

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

Eliminate the constructor from overload for invalid input values to help produce more meaningful diagnostics.

---
Full diff: https://github.com/llvm/llvm-project/pull/138456.diff


1 Files Affected:

- (modified) flang/include/flang/Evaluate/constant.h (+5-1) 


``````````diff
diff --git a/flang/include/flang/Evaluate/constant.h b/flang/include/flang/Evaluate/constant.h
index 6fc22e3b86aa2..d4c6601c37bca 100644
--- 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(

``````````

</details>


https://github.com/llvm/llvm-project/pull/138456