Index: gcc/builtin-types.def
===================================================================
--- gcc/builtin-types.def	(revision 278603)
+++ gcc/builtin-types.def	(working copy)
@@ -136,12 +136,24 @@ DEF_PRIMITIVE_TYPE (BT_WINT, wint_type_node)
 DEF_PRIMITIVE_TYPE (BT_STRING, string_type_node)
 DEF_PRIMITIVE_TYPE (BT_CONST_STRING, const_string_type_node)
 
-DEF_PRIMITIVE_TYPE (BT_DFLOAT32, dfloat32_type_node)
-DEF_PRIMITIVE_TYPE (BT_DFLOAT64, dfloat64_type_node)
-DEF_PRIMITIVE_TYPE (BT_DFLOAT128, dfloat128_type_node)
-DEF_PRIMITIVE_TYPE (BT_DFLOAT32_PTR, dfloat32_ptr_type_node)
-DEF_PRIMITIVE_TYPE (BT_DFLOAT64_PTR, dfloat64_ptr_type_node)
-DEF_PRIMITIVE_TYPE (BT_DFLOAT128_PTR, dfloat128_ptr_type_node)
+DEF_PRIMITIVE_TYPE (BT_DFLOAT32, (dfloat32_type_node
+				  ? dfloat32_type_node
+				  : error_mark_node))
+DEF_PRIMITIVE_TYPE (BT_DFLOAT64, (dfloat64_type_node
+				  ? dfloat64_type_node
+				  : error_mark_node))
+DEF_PRIMITIVE_TYPE (BT_DFLOAT128, (dfloat128_type_node
+				   ? dfloat128_type_node
+				   : error_mark_node))
+DEF_PRIMITIVE_TYPE (BT_DFLOAT32_PTR, (dfloat32_ptr_type_node
+				      ? dfloat32_ptr_type_node
+				      : error_mark_node))
+DEF_PRIMITIVE_TYPE (BT_DFLOAT64_PTR, (dfloat64_ptr_type_node
+				      ? dfloat64_ptr_type_node
+				      : error_mark_node))
+DEF_PRIMITIVE_TYPE (BT_DFLOAT128_PTR, (dfloat128_ptr_type_node
+				       ? dfloat128_ptr_type_node
+				       : error_mark_node))
 
 DEF_PRIMITIVE_TYPE (BT_VALIST_REF, va_list_ref_type_node)
 DEF_PRIMITIVE_TYPE (BT_VALIST_ARG, va_list_arg_type_node)
Index: gcc/c/c-decl.c
===================================================================
--- gcc/c/c-decl.c	(revision 278603)
+++ gcc/c/c-decl.c	(working copy)
@@ -11619,7 +11619,9 @@ finish_declspecs (struct c_declspecs *specs)
     case cts_dfloat128:
       gcc_assert (!specs->long_p && !specs->long_long_p && !specs->short_p
 		  && !specs->signed_p && !specs->unsigned_p && !specs->complex_p);
-      if (specs->typespec_word == cts_dfloat32)
+      if (!targetm.decimal_float_supported_p ())
+	specs->type = integer_type_node;
+      else if (specs->typespec_word == cts_dfloat32)
 	specs->type = dfloat32_type_node;
       else if (specs->typespec_word == cts_dfloat64)
 	specs->type = dfloat64_type_node;
Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c	(revision 278603)
+++ gcc/c-family/c-common.c	(working copy)
@@ -2321,11 +2321,14 @@ c_common_type_for_mode (machine_mode mode, int uns
 	return build_vector_type_for_mode (inner_type, mode);
     }
 
-  if (mode == TYPE_MODE (dfloat32_type_node))
+  if (dfloat32_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat32_type_node))
     return dfloat32_type_node;
-  if (mode == TYPE_MODE (dfloat64_type_node))
+  if (dfloat64_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat64_type_node))
     return dfloat64_type_node;
-  if (mode == TYPE_MODE (dfloat128_type_node))
+  if (dfloat128_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat128_type_node))
     return dfloat128_type_node;
 
   if (ALL_SCALAR_FIXED_POINT_MODE_P (mode))
Index: gcc/c-family/c-format.c
===================================================================
--- gcc/c-family/c-format.c	(revision 278603)
+++ gcc/c-family/c-format.c	(working copy)
@@ -4390,7 +4390,7 @@ get_format_for_type_1 (const format_kind_info *fki
       for (int i = 0; i < FMT_LEN_MAX; i++)
 	{
 	  const format_type_detail *ftd = &spec->types[i];
-	  if (!ftd->type)
+	  if (!ftd->type || *ftd->type == NULL_TREE)
 	    continue;
 	  if (matching_type_p (*ftd->type, effective_arg_type))
 	    {
Index: gcc/c-family/c-lex.c
===================================================================
--- gcc/c-family/c-lex.c	(revision 278603)
+++ gcc/c-family/c-lex.c	(working copy)
@@ -877,7 +877,12 @@ interpret_float (const cpp_token *token, unsigned
 
   /* Decode type based on width and properties. */
   if (flags & CPP_N_DFLOAT)
-    if ((flags & CPP_N_WIDTH) == CPP_N_LARGE)
+    if (!targetm.decimal_float_supported_p ())
+      {
+	error ("decimal floating-point not supported for this target");
+	return error_mark_node;
+      }
+    else if ((flags & CPP_N_WIDTH) == CPP_N_LARGE)
       type = dfloat128_type_node;
     else if ((flags & CPP_N_WIDTH) == CPP_N_SMALL)
       type = dfloat32_type_node;
Index: gcc/lto/lto-lang.c
===================================================================
--- gcc/lto/lto-lang.c	(revision 278603)
+++ gcc/lto/lto-lang.c	(working copy)
@@ -1051,11 +1051,14 @@ lto_type_for_mode (machine_mode mode, int unsigned
 	return build_vector_type_for_mode (inner_type, mode);
     }
 
-  if (mode == TYPE_MODE (dfloat32_type_node))
+  if (dfloat32_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat32_type_node))
     return dfloat32_type_node;
-  if (mode == TYPE_MODE (dfloat64_type_node))
+  if (dfloat64_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat64_type_node))
     return dfloat64_type_node;
-  if (mode == TYPE_MODE (dfloat128_type_node))
+  if (dfloat128_type_node != NULL_TREE
+      && mode == TYPE_MODE (dfloat128_type_node))
     return dfloat128_type_node;
 
   if (ALL_SCALAR_FIXED_POINT_MODE_P (mode))
Index: gcc/testsuite/gcc.dg/c2x-no-dfp-1.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-no-dfp-1.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-no-dfp-1.c	(working copy)
@@ -0,0 +1,12 @@
+/* Test DFP types and constants rejected if no DFP support.  Bug
+   91985.  */
+/* { dg-do compile { target { ! dfp } } } */
+/* { dg-options "-std=c2x" } */
+
+_Decimal32 d32a; /* { dg-error "not supported" } */
+_Decimal64 d64a; /* { dg-error "not supported" } */
+_Decimal128 d128a; /* { dg-error "not supported" } */
+
+_Bool d32b = 1.0DF; /* { dg-error "not supported" } */
+_Bool d64b = 1.0DD; /* { dg-error "not supported" } */
+_Bool d128b = 1.0DL; /* { dg-error "not supported" } */
Index: gcc/testsuite/gcc.dg/fltconst-pedantic-dfp.c
===================================================================
--- gcc/testsuite/gcc.dg/fltconst-pedantic-dfp.c	(revision 278603)
+++ gcc/testsuite/gcc.dg/fltconst-pedantic-dfp.c	(working copy)
@@ -2,5 +2,8 @@
 /* { dg-options "-pedantic" } */
 
 double a = 1.dl;	/* { dg-warning "decimal float" } */
+/* { dg-error "not supported for this target" "not supported" { target { ! dfp } } .-1 } */
 double b = 1.df;	/* { dg-warning "decimal float" } */
+/* { dg-error "not supported for this target" "not supported" { target { ! dfp } } .-1 } */
 double c = 1.dd;	/* { dg-warning "decimal float" } */
+/* { dg-error "not supported for this target" "not supported" { target { ! dfp } } .-1 } */
Index: gcc/testsuite/gcc.dg/gnu2x-builtins-no-dfp-1.c
===================================================================
--- gcc/testsuite/gcc.dg/gnu2x-builtins-no-dfp-1.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/gnu2x-builtins-no-dfp-1.c	(working copy)
@@ -0,0 +1,18 @@
+/* Test C2x built-in functions: test DFP built-in functions are not
+   available when no DFP support.  Bug 91985.  */
+/* { dg-do compile { target { ! dfp } } } */
+/* { dg-options "-std=gnu2x" } */
+
+int fabsd32 (void);
+int fabsd64 (void);
+int fabsd128 (void);
+int nand32 (void);
+int nand64 (void);
+int nand128 (void);
+
+__typeof__ (__builtin_fabsd32 (0)) d32; /* { dg-warning "implicit" } */
+__typeof__ (__builtin_fabsd64 (0)) d64; /* { dg-warning "implicit" } */
+__typeof__ (__builtin_fabsd128 (0)) d128; /* { dg-warning "implicit" } */
+__typeof__ (__builtin_nand32 (0)) d32n; /* { dg-warning "implicit" } */
+__typeof__ (__builtin_nand64 (0)) d64n; /* { dg-warning "implicit" } */
+__typeof__ (__builtin_nand128 (0)) d128n; /* { dg-warning "implicit" } */
Index: gcc/tree.c
===================================================================
--- gcc/tree.c	(revision 278603)
+++ gcc/tree.c	(working copy)
@@ -10334,23 +10334,26 @@ build_common_tree_nodes (bool signed_char)
   uint64_type_node = make_or_reuse_type (64, 1);
 
   /* Decimal float types. */
-  dfloat32_type_node = make_node (REAL_TYPE);
-  TYPE_PRECISION (dfloat32_type_node) = DECIMAL32_TYPE_SIZE;
-  SET_TYPE_MODE (dfloat32_type_node, SDmode);
-  layout_type (dfloat32_type_node);
-  dfloat32_ptr_type_node = build_pointer_type (dfloat32_type_node);
+  if (targetm.decimal_float_supported_p ())
+    {
+      dfloat32_type_node = make_node (REAL_TYPE);
+      TYPE_PRECISION (dfloat32_type_node) = DECIMAL32_TYPE_SIZE;
+      SET_TYPE_MODE (dfloat32_type_node, SDmode);
+      layout_type (dfloat32_type_node);
+      dfloat32_ptr_type_node = build_pointer_type (dfloat32_type_node);
 
-  dfloat64_type_node = make_node (REAL_TYPE);
-  TYPE_PRECISION (dfloat64_type_node) = DECIMAL64_TYPE_SIZE;
-  SET_TYPE_MODE (dfloat64_type_node, DDmode);
-  layout_type (dfloat64_type_node);
-  dfloat64_ptr_type_node = build_pointer_type (dfloat64_type_node);
+      dfloat64_type_node = make_node (REAL_TYPE);
+      TYPE_PRECISION (dfloat64_type_node) = DECIMAL64_TYPE_SIZE;
+      SET_TYPE_MODE (dfloat64_type_node, DDmode);
+      layout_type (dfloat64_type_node);
+      dfloat64_ptr_type_node = build_pointer_type (dfloat64_type_node);
 
-  dfloat128_type_node = make_node (REAL_TYPE);
-  TYPE_PRECISION (dfloat128_type_node) = DECIMAL128_TYPE_SIZE;
-  SET_TYPE_MODE (dfloat128_type_node, TDmode);
-  layout_type (dfloat128_type_node);
-  dfloat128_ptr_type_node = build_pointer_type (dfloat128_type_node);
+      dfloat128_type_node = make_node (REAL_TYPE);
+      TYPE_PRECISION (dfloat128_type_node) = DECIMAL128_TYPE_SIZE;
+      SET_TYPE_MODE (dfloat128_type_node, TDmode);
+      layout_type (dfloat128_type_node);
+      dfloat128_ptr_type_node = build_pointer_type (dfloat128_type_node);
+    }
 
   complex_integer_type_node = build_complex_type (integer_type_node, true);
   complex_float_type_node = build_complex_type (float_type_node, true);
