[flang] Complex constructors are scalar only

[klausler]

Summary

The common language extension that allows arbitary expressions
to be used as components in a complex constructor (x,y) -- not both
constant, since that would make it a complex literal constant --
still have to be scalar; it's not an elemental operation like the
CMPLX() intrinsic function is.

flang/lib/Semantics/expression.cpp

     const parser::Expr::ComplexConstructor &x) {
   auto re{Analyze(std::get<0>(x.t).value())};
   auto im{Analyze(std::get<1>(x.t).value())};
+  if (re && re->Rank() > 0) {
+    context().Say(std::get<0>(x.t).value().source,
+        "Real part of complex constructor must be scalar"_err_en_US);
+  }
+  if (im && im->Rank() > 0) {
+    context().Say(std::get<1>(x.t).value().source,
+        "Imaginary part of complex constructor must be scalar"_err_en_US);
+  }
   if (re && im) {
     ConformabilityCheck(GetContextualMessages(), *re, *im);
   }

flang/test/Semantics/expr-errors05.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! The components of a complex constructor (extension) must be scalar
+!ERROR: Real part of complex constructor must be scalar
+complex, parameter :: z1(*) = ([1.,2.], 3.)
+!ERROR: Imaginary part of complex constructor must be scalar
+complex, parameter :: z2(*) = (4., [5.,6.])
+end