[flang] Catch obscure structure constructor error

[klausler]

Summary

A scalar value in a structure constructor may correspond to an
array component in the derived type only when that component has
a shape to which the scalar value may be expanded.

flang/lib/Semantics/expression.cpp

                         "component", "value")};
                 if (checked && *checked && GetRank(*componentShape) > 0 &&
                     GetRank(*valueShape) == 0 &&
-                    !IsExpandableScalar(*converted, GetFoldingContext(),
-                        *componentShape, true /*admit PURE call*/)) {
+                    (IsDeferredShape(*symbol) ||
+                        !IsExpandableScalar(*converted, GetFoldingContext(),
+                            *componentShape, true /*admit PURE call*/))) {
                   AttachDeclaration(
                       Say(expr.source,
                           "Scalar value cannot be expanded to shape of array component '%s'"_err_en_US,

flang/test/Semantics/data01.f90

   integer :: myAge = 2
   type(person) associated
   type hasAlloc
-    integer, allocatable :: a(:)
+    integer, allocatable :: a
   end type
 end
 

flang/test/Semantics/structconst06.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Don't expand scalars for allocatable components.
+module m
+  type t
+    real, allocatable :: a(:)
+  end type
+  !ERROR: Scalar value cannot be expanded to shape of array component 'a'
+  type(t) :: x = t(0.)
+end module