Fix #493 - reading from a default unit (#521)

Author: vdonaldson

flang/lib/Lower/IO.cpp

@@ -1200,8 +1200,11 @@ mlir::Value getIOUnit(Fortran::lower::AbstractConverter &converter,
                       mlir::Location loc, const A &stmt, mlir::Type ty) {
   if (stmt.iounit)
     return genIOUnit(converter, loc, *stmt.iounit, ty);
-  return genIOUnit(converter, loc, *getIOControl<Fortran::parser::IoUnit>(stmt),
-                   ty);
+  if (auto *iounit = getIOControl<Fortran::parser::IoUnit>(stmt))
+    return genIOUnit(converter, loc, *iounit, ty);
+  auto &builder = converter.getFirOpBuilder();
+  return builder.create<mlir::ConstantOp>(
+      loc, builder.getIntegerAttr(ty, Fortran::runtime::io::DefaultUnit));
 }
 
 //===----------------------------------------------------------------------===//

flang/test/Lower/io-stmt02.f90

@@ -80,3 +80,15 @@ subroutine control1(n) ! I/O condition specifier control flow
 dimension c(n), d(n,n), e(n,n), f(n)
 read(*,'(F7.2)', iostat=mm, advance='no') a, b, (c(j), (d(k,j), e(k,j), k=1,n), f(j), j=1,n), g
 end
+
+! CHECK-LABEL: func @_QPimpliedformat
+subroutine impliedformat
+  ! CHECK: BeginExternalListInput(%c-1
+  ! CHECK: InputReal32
+  ! CHECK: EndIoStatement(%3) : (!fir.ref<i8>) -> i32
+  read*, x
+  ! CHECK: BeginExternalListOutput(%c-1
+  ! CHECK: OutputReal32
+  ! CHECK: EndIoStatement
+  print*, x
+end