diff --git a/gcc/fortran/io.c b/gcc/fortran/io.c

index d93dcfa..f47565c 100644

--- a/gcc/fortran/io.c

+++ b/gcc/fortran/io.c

@@ -909,6 +909,13 @@ data_desc:

       if (u != FMT_POSINT)

 	{

+	  if (flag_dec)

+	    {

+	      /* Assume a default width based on the variable size.  */

+	      saved_token = u;

+	      break;

+	    }

+

 	  format_locus.nextc += format_string_pos;

 	  gfc_error ("Positive width required in format "

 			 "specifier %s at %L", token_to_string (t),

@@ -1030,6 +1037,13 @@ data_desc:

 	goto fail;

       if (t != FMT_ZERO && t != FMT_POSINT)

 	{

+	  if (flag_dec)

+	    {

+	      /* Assume the default width is expected here and continue lexing.  */

+	      value = 0; /* It doesn't matter what we set the value to here.  */

+	      saved_token = t;

+	      break;

+	    }

 	  error = nonneg_required;

 	  goto syntax;

 	}

@@ -1099,8 +1113,17 @@ data_desc:

 	goto fail;

       if (t != FMT_ZERO && t != FMT_POSINT)

 	{

-	  error = nonneg_required;

-	  goto syntax;

+	  if (flag_dec)

+	    {

+	      /* Assume the default width is expected here and continue lexing.  */

+	      value = 0; /* It doesn't matter what we set the value to here.  */

+	      saved_token = t;

+	    }

+	  else

+	    {

+	      error = nonneg_required;

+	      goto syntax;

+	    }

 	}

       else if (is_input && t == FMT_ZERO)

 	{

diff --git a/gcc/testsuite/gfortran.dg/fmt_f_default_field_width.f90 b/gcc/testsuite/gfortran.dg/fmt_f_default_field_width.f90

new file mode 100644

index 0000000..b087b8f

--- /dev/null

+++ b/gcc/testsuite/gfortran.dg/fmt_f_default_field_width.f90

@@ -0,0 +1,43 @@

+! { dg-do run }

+! { dg-options -fdec }

+!

+! Test case for the default field widths enabled by the -fdec-format-defaults flag.

+!

+! This feature is not part of any Fortran standard, but it is supported by the

+! Oracle Fortran compiler and others.

+!

+! libgfortran uses printf() internally to implement FORMAT. If you print float

+! values to a higher precision than the type can actually store, the results

+! are implementation dependent: some platforms print zeros, others print random

+! numbers. Don't depend on this behaviour in tests because they will not be

+! portable.

+

+    character(50) :: buffer

+

+    real*4 :: real_4

+    real*8 :: real_8

+    real*16 :: real_16

+    integer :: len

+

+    real_4 = 4.18

+    write(buffer, '(A, F, A)') ':',real_4,':'

+    print *,buffer

+    if (buffer.ne.":      4.1799998:") call abort

+

+    real_4 = 0.00000018

+    write(buffer, '(A, F, A)') ':',real_4,':'

+    print *,buffer

+    if (buffer.ne.":      0.0000002:") call abort

+

+    real_8 = 4.18

+    write(buffer, '(A, F, A)') ':',real_8,':'

+    print *,buffer

+    len = len_trim(buffer)

+    if (len /= 27) call abort

+

+    real_16 = 4.18

+    write(buffer, '(A, F, A)') ':',real_16,':'

+    print *,buffer

+    len = len_trim(buffer)

+    if (len /= 44) call abort

+end

diff --git a/gcc/testsuite/gfortran.dg/fmt_g_default_field_width.f90 b/gcc/testsuite/gfortran.dg/fmt_g_default_field_width.f90

new file mode 100644

index 0000000..3d3a476

--- /dev/null

+++ b/gcc/testsuite/gfortran.dg/fmt_g_default_field_width.f90

@@ -0,0 +1,48 @@

+! { dg-do run }

+! { dg-options -fdec }

+!

+! Test case for the default field widths enabled by the -fdec-format-defaults flag.

+!

+! This feature is not part of any Fortran standard, but it is supported by the

+! Oracle Fortran compiler and others.

+!

+! libgfortran uses printf() internally to implement FORMAT. If you print float

+! values to a higher precision than the type can actually store, the results

+! are implementation dependent: some platforms print zeros, others print random

+! numbers. Don't depend on this behaviour in tests because they will not be

+! portable.

+

+    character(50) :: buffer

+

+    real*4 :: real_4

+    real*8 :: real_8

+    real*16 :: real_16

+    integer :: len

+

+    real_4 = 4.18

+    write(buffer, '(A, G, A)') ':',real_4,':'

+    print *,buffer

+    if (buffer.ne.":   4.180000    :") call abort

+

+    real_4 = 0.00000018

+    write(buffer, '(A, G, A)') ':',real_4,':'

+    print *,buffer

+    if (buffer.ne.":  0.1800000E-06:") call abort

+

+    real_4 = 18000000.4

+    write(buffer, '(A, G, A)') ':',real_4,':'

+    print *,buffer

+    if (buffer.ne.":  0.1800000E+08:") call abort

+

+    real_8 = 4.18

+    write(buffer, '(A, G, A)') ':',real_8,':'

+    print *,buffer

+    len = len_trim(buffer)

+    if (len /= 27) call abort

+

+    real_16 = 4.18

+    write(buffer, '(A, G, A)') ':',real_16,':'

+    print *,buffer

+    len = len_trim(buffer)

+    if (len /= 44) call abort

+end

diff --git a/gcc/testsuite/gfortran.dg/fmt_i_default_field_width.f90 b/gcc/testsuite/gfortran.dg/fmt_i_default_field_width.f90

new file mode 100644

index 0000000..ac4e165

--- /dev/null

+++ b/gcc/testsuite/gfortran.dg/fmt_i_default_field_width.f90

@@ -0,0 +1,38 @@

+! { dg-do run }

+! { dg-options -fdec }

+!

+! Test case for the default field widths enabled by the -fdec-format-defaults flag.

+!

+! This feature is not part of any Fortran standard, but it is supported by the

+! Oracle Fortran compiler and others.

+

+    character(50) :: buffer

+    character(1) :: colon

+

+    integer*2 :: integer_2

+    integer*4 :: integer_4

+    integer*8 :: integer_8

+

+    write(buffer, '(A, I, A)') ':',12340,':'

+    print *,buffer

+    if (buffer.ne.":       12340:") call abort

+

+    read(buffer, '(A1, I, A1)') colon, integer_4, colon

+    if (integer_4.ne.12340) call abort

+

+    integer_2 = -99

+    write(buffer, '(A, I, A)') ':',integer_2,':'

+    print *,buffer

+    if (buffer.ne.":    -99:") call abort

+

+    integer_8 = -11112222

+    write(buffer, '(A, I, A)') ':',integer_8,':'

+    print *,buffer

+    if (buffer.ne.":              -11112222:") call abort

+

+! If the width is 7 and there are 7 leading zeroes, the result should be zero.

+    integer_2 = 789

+    buffer = '0000000789'

+    read(buffer, '(I)') integer_2

+    if (integer_2.ne.0) call abort

+end

diff --git a/libgfortran/io/format.c b/libgfortran/io/format.c

index c2abdd7..692b1ff 100644

--- a/libgfortran/io/format.c

+++ b/libgfortran/io/format.c

@@ -956,12 +956,33 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)

 	  *seen_dd = true;

 	  if (u != FMT_POSINT && u != FMT_ZERO)

 	    {

+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)

+		{

+		  tail->u.real.w = DEFAULT_WIDTH;

+		  tail->u.real.d = 0;

+		  tail->u.real.e = -1;

+		  fmt->saved_token = u;

+		  break;

+		}

 	      fmt->error = nonneg_required;

 	      goto finished;

 	    }

 	}

+      else if (u == FMT_ZERO)

+	{

+	  fmt->error = posint_required;

+	  goto finished;

+	}

       else if (u != FMT_POSINT)

 	{

+	  if (dtp->common.flags & IOPARM_DT_DEC_EXT)

+	    {

+	      tail->u.real.w = DEFAULT_WIDTH;

+	      tail->u.real.d = 0;

+	      tail->u.real.e = -1;

+	      fmt->saved_token = u;

+	      break;

+	    }

 	  fmt->error = posint_required;

 	  goto finished;

 	}

@@ -1099,6 +1120,13 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)

 	{

 	  if (t != FMT_POSINT)

 	    {

+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)

+		{

+		  tail->u.integer.w = DEFAULT_WIDTH;

+		  tail->u.integer.m = -1;

+		  fmt->saved_token = t;

+		  break;

+		}

 	      fmt->error = posint_required;

 	      goto finished;

 	    }

@@ -1107,6 +1135,13 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)

 	{

 	  if (t != FMT_ZERO && t != FMT_POSINT)

 	    {

+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)

+		{

+		  tail->u.integer.w = DEFAULT_WIDTH;

+		  tail->u.integer.m = -1;

+		  fmt->saved_token = t;

+		  break;

+		}

 	      fmt->error = nonneg_required;

 	      goto finished;

 	    }

diff --git a/libgfortran/io/io.h b/libgfortran/io/io.h

index 5583183..d1d08e8 100644

--- a/libgfortran/io/io.h

+++ b/libgfortran/io/io.h

@@ -981,5 +981,55 @@ memset4 (gfc_char4_t *p, gfc_char4_t c, int k)

     *p++ = c;

 }

+/* Used in width fields to indicate that the default should be used */

+#define DEFAULT_WIDTH -1

+

+/* Defaults for certain format field descriptors. These are decided based on

+ * the type of the value being formatted.

+ *

+ * The behaviour here is modelled on the Oracle Fortran compiler. At the time

+ * of writing, the details were available at this URL:

+ *

+ *   https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vnc3/index.html#z4000743746d

+ */

+

+static inline int

+default_width_for_integer (int kind)

+{

+  switch (kind)

+    {

+    case 1:

+    case 2:  return  7;

+    case 4:  return 12;

+    case 8:  return 23;

+    case 16: return 44;

+    default: return  0;

+    }

+}

+

+static inline int

+default_width_for_float (int kind)

+{

+  switch (kind)

+    {

+    case 4:  return 15;

+    case 8:  return 25;

+    case 16: return 42;

+    default: return  0;

+    }

+}

+

+static inline int

+default_precision_for_float (int kind)

+{

+  switch (kind)

+    {

+    case 4:  return 7;

+    case 8:  return 16;

+    case 16: return 33;

+    default: return 0;

+    }

+}

+

 #endif

diff --git a/libgfortran/io/read.c b/libgfortran/io/read.c

index 2c9de48..e911e35 100644

--- a/libgfortran/io/read.c

+++ b/libgfortran/io/read.c

@@ -629,6 +629,12 @@ read_decimal (st_parameter_dt *dtp, const fnode *f, char *dest, int length)

   w = f->u.w;

+  /* This is a legacy extension, and the frontend will only allow such cases

+   * through when -fdec-format-defaults is passed.

+   */

+  if (w == DEFAULT_WIDTH)

+    w = default_width_for_integer (length);

+

   p = read_block_form (dtp, &w);

   if (p == NULL)

diff --git a/libgfortran/io/write.c b/libgfortran/io/write.c

index a7307a8..c8e52fb 100644

--- a/libgfortran/io/write.c

+++ b/libgfortran/io/write.c

@@ -684,9 +684,8 @@ write_l (st_parameter_dt *dtp, const fnode *f, char *source, int len)

   p[wlen - 1] = (n) ? 'T' : 'F';

 }

-

 static void

-write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n)

+write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n, int len)

 {

   int w, m, digits, nzero, nblank;

   char *p;

@@ -719,6 +718,9 @@ write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n)

   /* Select a width if none was specified.  The idea here is to always

      print something.  */

+  if (w == DEFAULT_WIDTH)

+    w = default_width_for_integer (len);

+

   if (w == 0)

     w = ((digits < m) ? m : digits);

@@ -845,6 +847,8 @@ write_decimal (st_parameter_dt *dtp, const fnode *f, const char *source,

   /* Select a width if none was specified.  The idea here is to always

      print something.  */

+  if (w == DEFAULT_WIDTH)

+    w = default_width_for_integer (len);

   if (w == 0)

     w = ((digits < m) ? m : digits) + nsign;

@@ -1187,13 +1191,13 @@ write_b (st_parameter_dt *dtp, const fnode *f, const char *source, int len)

   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))

     {

       p = btoa_big (source, itoa_buf, len, &n);

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

   else

     {

       n = extract_uint (source, len);

       p = btoa (n, itoa_buf, sizeof (itoa_buf));

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

 }

@@ -1208,13 +1212,13 @@ write_o (st_parameter_dt *dtp, const fnode *f, const char *source, int len)

   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))

     {

       p = otoa_big (source, itoa_buf, len, &n);

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

   else

     {

       n = extract_uint (source, len);

       p = otoa (n, itoa_buf, sizeof (itoa_buf));

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

 }

@@ -1228,13 +1232,13 @@ write_z (st_parameter_dt *dtp, const fnode *f, const char *source, int len)

   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))

     {

       p = ztoa_big (source, itoa_buf, len, &n);

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

   else

     {

       n = extract_uint (source, len);

       p = gfc_xtoa (n, itoa_buf, sizeof (itoa_buf));

-      write_boz (dtp, f, p, n);

+      write_boz (dtp, f, p, n, len);

     }

 }

@@ -1504,7 +1508,7 @@ size_from_kind (st_parameter_dt *dtp, const fnode *f, int kind)

 {

   int size;

-  if (f->format == FMT_F && f->u.real.w == 0)

+  if ((f->format == FMT_F && f->u.real.w == 0) || f->u.real.w == DEFAULT_WIDTH)

     {

       switch (kind)

       {

diff --git a/libgfortran/io/write_float.def b/libgfortran/io/write_float.def

index 7f0aa1d..73dc910 100644

--- a/libgfortran/io/write_float.def

+++ b/libgfortran/io/write_float.def

@@ -113,7 +113,8 @@ determine_precision (st_parameter_dt * d

 static void

 build_float_string (st_parameter_dt *dtp, const fnode *f, char *buffer,

 		    size_t size, int nprinted, int precision, int sign_bit,

-		    bool zero_flag, int npad, char *result, size_t *len)

+		    bool zero_flag, int npad, int default_width, char *result,

+                    size_t *len)

 {

   char *put;

   char *digits;

@@ -132,8 +133,17 @@ build_float_string (st_parameter_dt *dtp

   sign_t sign;

   ft = f->format;

-  w = f->u.real.w;

-  d = f->u.real.d;

+  if (f->u.real.w == DEFAULT_WIDTH)

+    /* This codepath can only be reached with -fdec-format-defaults. */

+    {

+      w = default_width;

+      d = precision;

+    }

+  else

+    {

+      w = f->u.real.w;

+      d = f->u.real.d;

+    }

   p = dtp->u.p.scale_factor;

   *len = 0;

@@ -959,6 +969,11 @@ determine_en_precision (st_parameter_dt

       int save_scale_factor;\

       volatile GFC_REAL_ ## x temp;\

       save_scale_factor = dtp->u.p.scale_factor;\

+      if (w == DEFAULT_WIDTH)\

+	{\

+	  w = default_width;\

+	  d = precision;\

+	}\

       switch (dtp->u.p.current_unit->round_status)\

 	{\

 	  case ROUND_ZERO:\

@@ -1034,7 +1049,8 @@ determine_en_precision (st_parameter_dt

 	  nprinted = FDTOA(y,precision,m);\

 	}\

       build_float_string (dtp, &newf, buffer, size, nprinted, precision,\

-				   sign_bit, zero_flag, npad, result, res_len);\

+				   sign_bit, zero_flag, npad, default_width,\

+				   result, res_len);\

       dtp->u.p.scale_factor = save_scale_factor;\

     }\

   else\

@@ -1044,7 +1060,8 @@ determine_en_precision (st_parameter_dt

       else\

 	nprinted = DTOA(y,precision,m);\

       build_float_string (dtp, f, buffer, size, nprinted, precision,\

-				   sign_bit, zero_flag, npad, result, res_len);\

+				   sign_bit, zero_flag, npad, default_width,\

+				   result, res_len);\

     }\

 }\

@@ -1058,6 +1075,16 @@ get_float_string (st_parameter_dt *dtp,

 {

   int sign_bit, nprinted;

   bool zero_flag;

+  int default_width = 0;

+

+  if (f->u.real.w == DEFAULT_WIDTH)

+    /* This codepath can only be reached with -fdec-format-defaults. The default

+     * values are based on those used in the Oracle Fortran compiler.

+     */

+    {

+      default_width = default_width_for_float (kind);

+      precision = default_precision_for_float (kind);

+    }

   switch (kind)

     {