diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 1d87845a882..aa1fd0c03c8 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,14 @@
+2017-01-07  Jason Merrill  <jason@redhat.com>
+
+	PR c++/78948 - instantiation from discarded statement
+	* parser.h (struct cp_parser): Remove in_discarded_stmt field.
+	* cp-tree.h (in_discarded_stmt): Declare it.
+	(struct saved_scope): Add discarded_stmt bitfield.
+	(in_discarded_stmt): New macro.
+	* decl2.c (mark_used): Check it.
+	* parser.c (cp_parser_selection_statement): Adjust.
+	(cp_parser_jump_statement): Adjust.
+
 2017-01-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/78931
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 39f5d790cdc..24de3462bb7 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1281,6 +1281,10 @@ struct GTY(()) saved_scope {
   BOOL_BITFIELD x_processing_explicit_instantiation : 1;
   BOOL_BITFIELD need_pop_function_context : 1;
 
+/* Nonzero if we are parsing the discarded statement of a constexpr
+   if-statement.  */
+  BOOL_BITFIELD discarded_stmt : 1;
+
   int unevaluated_operand;
   int inhibit_evaluation_warnings;
   int noexcept_operand;
@@ -1341,6 +1345,8 @@ extern GTY(()) struct saved_scope *scope_chain;
 #define processing_specialization scope_chain->x_processing_specialization
 #define processing_explicit_instantiation scope_chain->x_processing_explicit_instantiation
 
+#define in_discarded_stmt scope_chain->discarded_stmt
+
 /* RAII sentinel to handle clearing processing_template_decl and restoring
    it when done.  */
 
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index a0375ad4cc4..435f51fe564 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -5112,7 +5112,7 @@ mark_used (tree decl, tsubst_flags_t complain)
     }
 
   /* If we don't need a value, then we don't need to synthesize DECL.  */
-  if (cp_unevaluated_operand != 0)
+  if (cp_unevaluated_operand || in_discarded_stmt)
     return true;
 
   DECL_ODR_USED (decl) = 1;
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 57ae0647195..e8c06424e29 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -11147,12 +11147,12 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 
 	    /* Outside a template, the non-selected branch of a constexpr
 	       if is a 'discarded statement', i.e. unevaluated.  */
-	    bool was_discarded = parser->in_discarded_stmt;
+	    bool was_discarded = in_discarded_stmt;
 	    bool discard_then = (cx && !processing_template_decl
 				 && integer_zerop (condition));
 	    if (discard_then)
 	      {
-		parser->in_discarded_stmt = true;
+		in_discarded_stmt = true;
 		++c_inhibit_evaluation_warnings;
 	      }
 
@@ -11166,7 +11166,7 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 	    if (discard_then)
 	      {
 		THEN_CLAUSE (statement) = NULL_TREE;
-		parser->in_discarded_stmt = was_discarded;
+		in_discarded_stmt = was_discarded;
 		--c_inhibit_evaluation_warnings;
 	      }
 
@@ -11178,7 +11178,7 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 				     && integer_nonzerop (condition));
 		if (discard_else)
 		  {
-		    parser->in_discarded_stmt = true;
+		    in_discarded_stmt = true;
 		    ++c_inhibit_evaluation_warnings;
 		  }
 
@@ -11235,7 +11235,7 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 		if (discard_else)
 		  {
 		    ELSE_CLAUSE (statement) = NULL_TREE;
-		    parser->in_discarded_stmt = was_discarded;
+		    in_discarded_stmt = was_discarded;
 		    --c_inhibit_evaluation_warnings;
 		  }
 	      }
@@ -12143,7 +12143,7 @@ cp_parser_jump_statement (cp_parser* parser)
 	     expression.  */
 	  expr = NULL_TREE;
 	/* Build the return-statement.  */
-	if (current_function_auto_return_pattern && parser->in_discarded_stmt)
+	if (current_function_auto_return_pattern && in_discarded_stmt)
 	  /* Don't deduce from a discarded return statement.  */;
 	else
 	  statement = finish_return_stmt (expr);
diff --git a/gcc/cp/parser.h b/gcc/cp/parser.h
index f242f4c6153..0994e1e7f4f 100644
--- a/gcc/cp/parser.h
+++ b/gcc/cp/parser.h
@@ -336,10 +336,6 @@ struct GTY(()) cp_parser {
      a local class.  */
   bool in_function_body;
 
-  /* TRUE if we are parsing a C++17 discarded statement (the non-taken branch
-     of an if constexpr).  */
-  bool in_discarded_stmt;
-
   /* Nonzero if we're processing a __transaction_atomic or
      __transaction_relaxed statement.  */
   unsigned char in_transaction;
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if10.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if10.C
new file mode 100644
index 00000000000..64de53f0603
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if10.C
@@ -0,0 +1,16 @@
+// PR c++/79848
+// { dg-options -std=c++1z }
+
+template <int T>
+void sizeof_mismatch()
+{
+    static_assert(T == 0, "sizeof mismatch");
+}
+
+int main()
+{
+  if constexpr(sizeof(long long) == sizeof(char*))
+    ;
+  else
+    sizeof_mismatch<sizeof(long long)>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if11.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if11.C
new file mode 100644
index 00000000000..1c6247e0e35
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if11.C
@@ -0,0 +1,16 @@
+// Test that discarded statements differ from unevaluated operands in some
+// ways.
+// { dg-options -std=c++1z }
+
+struct A { int i; };
+
+int main()
+{
+  if constexpr(true)
+    ;
+  else
+    {
+      []{}();
+      A::i;			// { dg-error "non-static" }
+    }
+}