[flang] Ensure that structure constructors fold parameter references

Structure contructors for instances of parameterized derived types
must have their components' values folded in the context of the values
of the type parameters.

Differential Revision: https://reviews.llvm.org/D125116

flang/lib/Evaluate/fold.cpp

@@ -67,6 +67,7 @@ Expr<SomeDerived> FoldOperation(
     FoldingContext &context, StructureConstructor &&structure) {
   StructureConstructor ctor{structure.derivedTypeSpec()};
   bool isConstant{true};
+  auto restorer{context.WithPDTInstance(structure.derivedTypeSpec())};
   for (auto &&[symbol, value] : std::move(structure)) {
     auto expr{Fold(context, std::move(value.value()))};
     if (IsPointer(symbol)) {

flang/test/Semantics/structconst05.f90

@@ -0,0 +1,9 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Ensure that PDT instance structure constructors can be folded to constants
+module m1
+  type :: pdt(k)
+    integer, len :: k
+    character(len=k) :: x, y = "def"
+  end type
+  type(pdt(4)) :: v = pdt(4)("abc")
+end module