[flang] Improve error message on bad complex literal. (#124331)

A complex literal constant can have one BOZ component, since the type
and value of the literal can be determined by converting the BOZ value
to the type of the other component. But a complex literal constant with
two BOZ components doesn't have a well-defined type. The error message
was confusing in the case; emit a better one.

Fixes #124201.

Author: klausler

flang/lib/Evaluate/tools.cpp

@@ -212,6 +212,11 @@ ConvertRealOperandsResult ConvertRealOperands(
             return {AsSameKindExprs<TypeCategory::Real>(
                 ConvertTo(ry, std::move(bx)), std::move(ry))};
           },
+          [&](BOZLiteralConstant &&,
+              BOZLiteralConstant &&) -> ConvertRealOperandsResult {
+            messages.Say("operands cannot both be BOZ"_err_en_US);
+            return std::nullopt;
+          },
           [&](auto &&, auto &&) -> ConvertRealOperandsResult { // C718
             messages.Say(
                 "operands must be INTEGER, UNSIGNED, REAL, or BOZ"_err_en_US);

flang/test/Semantics/complex01.f90

@@ -1,5 +1,5 @@
 ! RUN: %python %S/test_errors.py %s %flang_fc1
-! C718 Each named constant in a complex literal constant shall be of type 
+! C718 Each named constant in a complex literal constant shall be of type
 ! integer or real.
 subroutine s()
   integer :: ivar = 35
@@ -30,4 +30,6 @@ subroutine s()
   complex :: cvar11 = (cconst, 1.0)
   !ERROR: operands must be INTEGER, UNSIGNED, REAL, or BOZ
   complex :: cvar12 = (lconst, 1.0)
+  !ERROR: operands cannot both be BOZ
+  complex :: cvar13 = (z'3f700000', z'00000000')
 end subroutine s