diff --git gcc/c-family/c-cppbuiltin.c gcc/c-family/c-cppbuiltin.c
index 76d1e4a380e..9766d7f96c4 100644
--- gcc/c-family/c-cppbuiltin.c
+++ gcc/c-family/c-cppbuiltin.c
@@ -972,7 +972,8 @@ c_cpp_builtins (cpp_reader *pfile)
 	  cpp_define (pfile, "__cpp_fold_expressions=201603L");
 	  cpp_define (pfile, "__cpp_nontype_template_args=201411L");
 	  cpp_define (pfile, "__cpp_range_based_for=201603L");
-	  cpp_define (pfile, "__cpp_constexpr=201603L");
+	  if (cxx_dialect <= cxx17)
+	    cpp_define (pfile, "__cpp_constexpr=201603L");
 	  cpp_define (pfile, "__cpp_if_constexpr=201606L");
 	  cpp_define (pfile, "__cpp_capture_star_this=201603L");
 	  cpp_define (pfile, "__cpp_inline_variables=201606L");
@@ -991,6 +992,7 @@ c_cpp_builtins (cpp_reader *pfile)
 	{
 	  /* Set feature test macros for C++2a.  */
 	  cpp_define (pfile, "__cpp_conditional_explicit=201806L");
+	  cpp_define (pfile, "__cpp_constexpr=201907L");
 	  cpp_define (pfile, "__cpp_constinit=201907L");
 	  cpp_define (pfile, "__cpp_nontype_template_parameter_class=201806L");
 	  cpp_define (pfile, "__cpp_impl_destroying_delete=201806L");
diff --git gcc/cp/class.c gcc/cp/class.c
index ef1d5136963..1fd5a7864e5 100644
--- gcc/cp/class.c
+++ gcc/cp/class.c
@@ -5234,8 +5234,14 @@ trivial_default_constructor_is_constexpr (tree t)
   /* A defaulted trivial default constructor is constexpr
      if there is nothing to initialize.  */
   gcc_assert (!TYPE_HAS_COMPLEX_DFLT (t));
-  /* A class with a vptr doesn't have a trivial default ctor.  */
-  return is_really_empty_class (t, /*ignore_vptr*/true);
+  /* A class with a vptr doesn't have a trivial default ctor.
+     In C++20, a class can have transient uninitialized members, e.g.:
+
+       struct S { int i; constexpr S() = default; };
+
+     should work.  */
+  return (cxx_dialect >= cxx2a
+	  || is_really_empty_class (t, /*ignore_vptr*/true));
 }
 
 /* Returns true iff class T has a constexpr default constructor.  */
diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 8c79b0484fc..2161aa62d44 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -142,6 +142,7 @@ struct GTY((for_user)) constexpr_fundef {
   tree body;
   tree parms;
   tree result;
+  bool missing_init;
 };
 
 struct constexpr_fundef_hasher : ggc_ptr_hash<constexpr_fundef>
@@ -182,7 +183,7 @@ retrieve_constexpr_fundef (tree fun)
   if (constexpr_fundef_table == NULL)
     return NULL;
 
-  constexpr_fundef fundef = { fun, NULL, NULL, NULL };
+  constexpr_fundef fundef = { fun, NULL, NULL, NULL, false };
   return constexpr_fundef_table->find (&fundef);
 }
 
@@ -762,10 +763,13 @@ massage_constexpr_body (tree fun, tree body)
 }
 
 /* CTYPE is a type constructed from BODY.  Return true if some
-   bases/fields are uninitialized, and complain if COMPLAIN.  */
+   bases/fields are uninitialized, and complain if COMPLAIN.
+   In C++20, we allow missing field initializers.  In such a case
+   record that in MISSING_INIT, if non-null.  */
 
 static bool
-cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
+cx_check_missing_mem_inits (tree ctype, tree body, bool complain,
+			    bool *missing_init = NULL)
 {
   unsigned nelts = 0;
   
@@ -779,7 +783,9 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 
   if (TREE_CODE (ctype) == UNION_TYPE)
     {
-      if (nelts == 0 && next_initializable_field (field))
+      if (cxx_dialect < cxx2a
+	  && nelts == 0
+	  && next_initializable_field (field))
 	{
 	  if (complain)
 	    error ("%<constexpr%> constructor for union %qT must "
@@ -815,13 +821,19 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 	    continue;
 	  if (ANON_AGGR_TYPE_P (TREE_TYPE (field)))
 	    {
-	      /* Recurse to check the anonummous aggregate member.  */
+	      /* Recurse to check the anonymous aggregate member.  */
 	      bad |= cx_check_missing_mem_inits
-		(TREE_TYPE (field), NULL_TREE, complain);
+		(TREE_TYPE (field), NULL_TREE, complain, missing_init);
 	      if (bad && !complain)
 		return true;
 	      continue;
 	    }
+	  if (cxx_dialect >= cxx2a)
+	    {
+	      if (missing_init)
+		*missing_init = true;
+	      continue;
+	    }
 	  ftype = strip_array_types (TREE_TYPE (field));
 	  if (type_has_constexpr_default_constructor (ftype))
 	    {
@@ -847,7 +859,8 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 	{
 	  /* Check the anonymous aggregate initializer is valid.  */
 	  bad |= cx_check_missing_mem_inits
-	    (TREE_TYPE (index), CONSTRUCTOR_ELT (body, i)->value, complain);
+	    (TREE_TYPE (index), CONSTRUCTOR_ELT (body, i)->value, complain,
+	     missing_init);
 	  if (bad && !complain)
 	    return true;
 	}
@@ -888,9 +901,11 @@ register_constexpr_fundef (tree fun, tree body)
       return NULL;
     }
 
+  bool missing_init = false;
   if (DECL_CONSTRUCTOR_P (fun)
       && cx_check_missing_mem_inits (DECL_CONTEXT (fun),
-				     massaged, !DECL_GENERATED_P (fun)))
+				     massaged, !DECL_GENERATED_P (fun),
+				     &missing_init))
     return NULL;
 
   /* Create the constexpr function table if necessary.  */
@@ -908,6 +923,7 @@ register_constexpr_fundef (tree fun, tree body)
       DECL_CONTEXT (DECL_RESULT (fun)) = fun;
     }
   entry.body = copy_fn (fun, entry.parms, entry.result);
+  entry.missing_init = missing_init;
   current_function_decl = saved_fn;
   slot = constexpr_fundef_table->find_slot (&entry, INSERT);
   if (clear_ctx)
@@ -2149,9 +2165,24 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	entry->result = result;
     }
 
-  /* The result of a constexpr function must be completely initialized.  */
+  /* The result of a constexpr function must be completely initialized.
+     Except in C++20 where a constexpr constructor doesn't necessarily
+     have to initialize all the fields, so we set CONSTRUCTOR_NO_CLEARING
+     in order to detect reading an unitialized object in constexpr instead
+     of value-initializing it.  */
   if (TREE_CODE (result) == CONSTRUCTOR)
-    clear_no_implicit_zero (result);
+    {
+      /* But do clear the flag in nested constructors.  */
+      clear_no_implicit_zero (result);
+      if (cxx_dialect >= cxx2a
+	  && DECL_CONSTRUCTOR_P (fun)
+	  && DECL_DECLARED_CONSTEXPR_P (fun))
+	{
+	  constexpr_fundef *f = retrieve_constexpr_fundef (fun);
+	  if (f && f->missing_init)
+	    CONSTRUCTOR_NO_CLEARING (result) = true;
+	}
+    }
 
   pop_cx_call_context ();
   return result;
diff --git gcc/cp/decl.c gcc/cp/decl.c
index 86e38f4af69..ab6da6080a5 100644
--- gcc/cp/decl.c
+++ gcc/cp/decl.c
@@ -5835,8 +5835,12 @@ check_for_uninitialized_const_var (tree decl, bool constexpr_context_p,
      7.1.6 */
   if (VAR_P (decl)
       && !TYPE_REF_P (type)
-      && (constexpr_context_p
-	  || CP_TYPE_CONST_P (type) || var_in_constexpr_fn (decl))
+      && (CP_TYPE_CONST_P (type)
+	  /* C++20 permits trivial default initialization in constexpr
+	     context (P1331R2).  */
+	  || (cxx_dialect < cxx2a
+	      && (constexpr_context_p
+		  || var_in_constexpr_fn (decl))))
       && !DECL_NONTRIVIALLY_INITIALIZED_P (decl))
     {
       tree field = default_init_uninitialized_part (type);
@@ -5845,7 +5849,7 @@ check_for_uninitialized_const_var (tree decl, bool constexpr_context_p,
 
       bool show_notes = true;
 
-      if (!constexpr_context_p)
+      if (!constexpr_context_p || cxx_dialect >= cxx2a)
 	{
 	  if (CP_TYPE_CONST_P (type))
 	    {
diff --git gcc/cp/method.c gcc/cp/method.c
index acba6c6da8c..39782970716 100644
--- gcc/cp/method.c
+++ gcc/cp/method.c
@@ -1985,10 +1985,12 @@ walk_field_subobs (tree fields, special_function_kind sfk, tree fnname,
 	  if (bad && deleted_p)
 	    *deleted_p = true;
 
-	  /* For an implicitly-defined default constructor to be constexpr,
-	     every member must have a user-provided default constructor or
-	     an explicit initializer.  */
-	  if (constexpr_p && !CLASS_TYPE_P (mem_type)
+	  /* Before C++20, for an implicitly-defined default constructor to
+	     be constexpr, every member must have a user-provided default
+	     constructor or an explicit initializer.  */
+	  if (constexpr_p
+	      && cxx_dialect < cxx2a
+	      && !CLASS_TYPE_P (mem_type)
 	      && TREE_CODE (DECL_CONTEXT (field)) != UNION_TYPE)
 	    {
 	      *constexpr_p = false;
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-array6.C gcc/testsuite/g++.dg/cpp0x/constexpr-array6.C
index 16eacdde440..48dae5d9609 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-array6.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-array6.C
@@ -4,7 +4,7 @@
 struct A
 {
   int i;
-  constexpr A() {}		// { dg-error "A::i" }
+  constexpr A() {}		// { dg-error "A::i" "" { target c++17_down } }
 };
 
 struct B
@@ -12,4 +12,5 @@ struct B
   A a;
 };
 
-constexpr B b[] = { {} };	// { dg-error "A::A" }
+constexpr B b[] = { {} };	// { dg-error "A::A" "" { target c++17_down } }
+// { dg-error "is not a constant expression" "" { target c++2a } .-1 }
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-ctor.C gcc/testsuite/g++.dg/cpp0x/constexpr-ctor.C
index 55beda7c49f..1d0fa479cbc 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-ctor.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-ctor.C
@@ -3,5 +3,5 @@
 struct A
 {
   int i;
-  constexpr A() { }		// { dg-error "A::i" }
+  constexpr A() { }		// { dg-error "A::i" "" { target c++17_down } }
 };
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-diag3.C gcc/testsuite/g++.dg/cpp0x/constexpr-diag3.C
index e54b26c7f6a..1c43569615c 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-diag3.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-diag3.C
@@ -48,7 +48,7 @@ struct Def
 {
   int _M_i;			// { dg-message "does not initialize" }
 
-  constexpr Def() = default;	// { dg-error "implicit declaration is not .constexpr." }
+  constexpr Def() = default;	// { dg-error "implicit declaration is not .constexpr." "" { target c++17_down } }
 };
 
 constexpr Def defobj;		// { dg-error "uninitialized" }
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-diag4.C gcc/testsuite/g++.dg/cpp0x/constexpr-diag4.C
index 13ca6fa2390..c603bdd1a00 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-diag4.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-diag4.C
@@ -21,5 +21,5 @@ struct A1
 struct B1
 {
     A1 a1;
-    constexpr B1() {} // { dg-error "B1::a1" }
+    constexpr B1() {} // { dg-error "B1::a1" "" { target c++17_down } }
 };
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
index a5893563eec..9d6d5ff587c 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
@@ -6,7 +6,7 @@
 struct A
 {
   int i;
-  constexpr A(int _i) { i = _i; } // { dg-error "empty body|A::i" }
+  constexpr A(int _i) { i = _i; } // { dg-error "empty body|A::i" "" { target c++17_down } }
 };
 
 template <class T>
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-template2.C gcc/testsuite/g++.dg/cpp0x/constexpr-template2.C
index 12a8d42b31f..71eb559a24c 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-template2.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-template2.C
@@ -3,7 +3,7 @@
 template <class T> struct A
 {
   T t;
-  constexpr A() { }		// { dg-error "::t" }
+  constexpr A() { }		// { dg-error "::t" "" { target c++17_down } }
 };
 
 int main()
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-union2.C gcc/testsuite/g++.dg/cpp0x/constexpr-union2.C
index 1a5e832ac34..c22ecc99efb 100644
--- gcc/testsuite/g++.dg/cpp0x/constexpr-union2.C
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-union2.C
@@ -14,5 +14,5 @@ union bar
   int x;
   short y;
 
-  constexpr bar() = default;	// { dg-error "constexpr" }
+  constexpr bar() = default;	// { dg-error "constexpr" "" { target c++17_down } }
 };
diff --git gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle.C gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle.C
index 15b8b79ecbc..7894ef3051e 100644
--- gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle.C
+++ gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle.C
@@ -47,17 +47,6 @@ struct S {
 	 []{return 3;}());
 };
 
-template<typename T> struct R {
-  static int x;
-};
-// "int i;" makes the op() non-constexpr in C++17.
-template<typename T> int R<T>::x = []{int i; return 1;}();
-template int R<int>::x;
-// Type of lambda in intializer of R<int>::x: N1RIiE1xMUlvE_E
-// Corresponding operator(): _ZNK1RIiE1xMUlvE_clEv
-// { dg-final { scan-assembler "_ZNK1RIiE1xMUlvE_clEv" } }
-// { dg-final { scan-assembler "weak\[^\n\r\]*_?_ZNK1RIiE1xMUlvE_clEv" { target { ! { *-*-mingw* *-*-cygwin } } } } }
-
 void bar()
 {
   // lambdas in non-vague linkage functions have internal linkage.
diff --git gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle6.C gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle6.C
new file mode 100644
index 00000000000..9ec13e79bbf
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/lambda/lambda-mangle6.C
@@ -0,0 +1,15 @@
+// Test lambda mangling
+// { dg-do compile { target { c++11 && c++17_down } } }
+// { dg-require-weak "" }
+// { dg-options "-fno-inline" }
+
+template<typename T> struct R {
+  static int x;
+};
+// "int i;" makes the op() non-constexpr in C++17.  In C++20, it does not.
+template<typename T> int R<T>::x = []{int i; return 1;}();
+template int R<int>::x;
+// Type of lambda in intializer of R<int>::x: N1RIiE1xMUlvE_E
+// Corresponding operator(): _ZNK1RIiE1xMUlvE_clEv
+// { dg-final { scan-assembler "_ZNK1RIiE1xMUlvE_clEv" } }
+// { dg-final { scan-assembler "weak\[^\n\r\]*_?_ZNK1RIiE1xMUlvE_clEv" { target { ! { *-*-mingw* *-*-cygwin } } } } }
diff --git gcc/testsuite/g++.dg/cpp0x/pr79118.C gcc/testsuite/g++.dg/cpp0x/pr79118.C
index 5db22a96dd4..616b51ea29a 100644
--- gcc/testsuite/g++.dg/cpp0x/pr79118.C
+++ gcc/testsuite/g++.dg/cpp0x/pr79118.C
@@ -13,7 +13,7 @@ struct One
   constexpr One () : a(), b() {} // { dg-error "multiple" }
   constexpr One (int) : a() {}
   constexpr One (unsigned) : b () {}
-  constexpr One (void *) {} // { dg-error "exactly one" }
+  constexpr One (void *) {} // { dg-error "exactly one" "" { target c++17_down } }
 };
 
 One a ();
@@ -30,10 +30,10 @@ struct Two
   };
 
   constexpr Two () : a(), b() {}
-  constexpr Two (int) : a() {} // { dg-error "b' must be initialized" }
-  constexpr Two (unsigned) : b () {} // { dg-error "a' must be initialized" }
-  constexpr Two (void *) {} // { dg-error "a' must be initialized" }
-   // { dg-error "b' must be initialized" "" { target *-*-* } .-1 }
+  constexpr Two (int) : a() {} // { dg-error "b' must be initialized" "" { target c++17_down } }
+  constexpr Two (unsigned) : b () {} // { dg-error "a' must be initialized" "" { target c++17_down } }
+  constexpr Two (void *) {} // { dg-error "a' must be initialized" "" { target c++17_down } }
+   // { dg-error "b' must be initialized" "" { target c++17_down } .-1 }
 };
 
 Two e ();
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-83921-3.C gcc/testsuite/g++.dg/cpp1y/constexpr-83921-3.C
index 4b1ed5c3a87..2f1218693e0 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-83921-3.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-83921-3.C
@@ -2,4 +2,4 @@
 // { dg-do compile { target c++14 } }
 
 struct Foo { int m; };
-constexpr void test() { Foo f; }  // { dg-error "uninitialized" }
+constexpr void test() { Foo f; }  // { dg-error "uninitialized" "" { target c++17_down } }
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-neg1.C gcc/testsuite/g++.dg/cpp1y/constexpr-neg1.C
index d82dbada1bf..53f0f1f7a2b 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-neg1.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-neg1.C
@@ -8,7 +8,7 @@ constexpr int f(int i) {
   goto foo;			// { dg-error "goto" }
  foo:
   asm("foo");			// { dg-error "asm" "" { target c++17_down } }
-  int k;			// { dg-error "uninitialized" }
+  int k;			// { dg-error "uninitialized" "" { target c++17_down } }
   A a;				// { dg-error "non-literal" }
   return i;
 }
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-lambda12.C gcc/testsuite/g++.dg/cpp1z/constexpr-lambda12.C
index a59bf497b30..93b53273741 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-lambda12.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-lambda12.C
@@ -3,7 +3,7 @@
 void f(int i)
 {
   [i]() constexpr {
-    int j;			// { dg-error "uninitialized" }
+    int j;			// { dg-error "uninitialized" "" { target c++17_down } }
     j = i;
     return j;
   }();
diff --git gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
index a18d6aea985..aee1d692a4e 100644
--- gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
+++ gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
@@ -1,5 +1,4 @@
-// { dg-do compile { target c++17 } }
-// { dg-options "-I${srcdir}/g++.dg/cpp1y -I${srcdir}/g++.dg/cpp1y/testinc" }
+// { dg-options "-std=c++17 -I${srcdir}/g++.dg/cpp1y -I${srcdir}/g++.dg/cpp1y/testinc" }
 
 //  C++98 features:
 
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init1.C gcc/testsuite/g++.dg/cpp2a/constexpr-init1.C
new file mode 100644
index 00000000000..b2248006c03
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init1.C
@@ -0,0 +1,100 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+// Test basic use.
+
+struct S {
+  int i;
+  constexpr S(bool b) {
+    if (b)
+      i = 42;
+  }
+};
+constexpr S s1(true);
+constexpr S s2(false); // { dg-error "not a constant expression" }
+
+constexpr int
+fn1 (int x)
+{
+  int a;
+  a = 5;
+  return x + a;
+}
+
+static_assert (fn1 (2) == 7);
+
+constexpr int
+fn2 (int x)
+{
+  const int a; // { dg-error "uninitialized .const a." }
+  constexpr int b; // { dg-error "uninitialized .const b." }
+  return x;
+}
+
+constexpr int
+fn3 (int x)
+{
+  int a; // { dg-message ".int a. is not const" }
+  return x + a;
+}
+
+constexpr int a = fn3 (5); // { dg-error "the value of .a. is not usable in a constant expression" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
+
+constexpr int
+fn4 ()
+{
+  struct S { int a = -5; int b; } s;
+  return s.a;
+}
+
+static_assert (fn4 () == -5);
+
+constexpr int
+fn5 ()
+{
+  struct S { int a = 9; int b; } s;
+  return s.b;
+}
+
+constexpr int b = fn5 (); // { dg-error "accessing uninitialized member" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
+
+constexpr int
+fn6 ()
+{
+  int a;
+  return 42;
+}
+
+static_assert (fn6 () == 42);
+
+constexpr int
+fn7 (bool b)
+{
+  int a; // { dg-message ".int a. is not const" }
+  if (b)
+    a = 42;
+  return a;
+}
+
+static_assert (fn7 (true) == 42);
+static_assert (fn7 (false) == 42); // { dg-error "non-constant condition|the value of .a. is not usable" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
+
+constexpr int
+fn8 (int n)
+{
+  int r;
+  switch (n)
+    {
+    case 1:
+    r = n;
+    return r;
+    case 42:
+    r = n;
+    return r;
+    }
+}
+
+static_assert (fn8 (1) == 1);
+static_assert (fn8 (42) == 42);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init10.C gcc/testsuite/g++.dg/cpp2a/constexpr-init10.C
new file mode 100644
index 00000000000..0d7b724a040
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init10.C
@@ -0,0 +1,11 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+// ??? In c++2a we don't emit a call to _ZN3FooI3ArgEC1Ev.
+
+struct Arg;
+struct Base {
+  int i;
+  virtual ~Base();
+};
+template <class> struct Foo : Base { };
+Foo<Arg> a;
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init2.C gcc/testsuite/g++.dg/cpp2a/constexpr-init2.C
new file mode 100644
index 00000000000..541da1c023f
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init2.C
@@ -0,0 +1,15 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct A
+{
+  int i;
+  constexpr A() : i{} {} 
+};
+
+struct B
+{
+  A a;
+};
+
+constexpr B b[] = { {} };
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init3.C gcc/testsuite/g++.dg/cpp2a/constexpr-init3.C
new file mode 100644
index 00000000000..dd2735289cb
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init3.C
@@ -0,0 +1,16 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct A
+{
+  int i;
+  constexpr A() {} 
+};
+
+struct B
+{
+  A a;
+};
+
+// A::i not initialized.
+constexpr B b[] = { {} }; // { dg-error "is not a constant expression" }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init4.C gcc/testsuite/g++.dg/cpp2a/constexpr-init4.C
new file mode 100644
index 00000000000..dd614ede2c6
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init4.C
@@ -0,0 +1,61 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+// This bullet in [dcl.constexpr] is now gone:
+//  - every non-static data member and base class sub-object shall be initialized
+
+struct A {
+  int i;
+  constexpr A(int _i) { i = _i; }
+};
+
+struct B {
+  int i;
+  constexpr B() { }
+};
+
+// Anonymous members.
+struct E {
+  int a;
+  union {
+    char b;
+    __extension__ struct {
+      double c;
+      long d;
+    };  
+    union {
+      char e;
+      void *f; 
+    };  
+  };  
+  __extension__ struct {
+    long long g;
+    __extension__ struct {
+      int h;
+      double i;
+    };  
+    union {
+      char *j; 
+      E *k; 
+    };  
+  };  
+
+  // Completely initialized.
+  constexpr E(int(&)[1]) : a(), b(), g(), h(), i(), j() {}
+  constexpr E(int(&)[3]) : a(), e(), g(), h(), i(), k() {}
+  constexpr E(int(&)[7]) : a(), b(), g(), h(), i(), j() {}
+  constexpr E(int(&)[8]) : a(), f(), g(), h(), i(), k() {}
+  constexpr E(int(&)[9]) : a(), c(), d(), g(), h(), i(), k() {}
+
+  // Missing d, i, j/k union init.
+  constexpr E(int(&)[2]) : a(), c(), g(), h() {}
+
+  // Missing h, j/k union init.
+  constexpr E(int(&)[4]) : a(), c(), d(), g(), i() {}
+
+  // Missing b/c/d/e/f union init.
+  constexpr E(int(&)[5]) : a(), g(), h(), i(), k() {}
+
+  // Missing a, b/c/d/e/f union, g/h/i/j/k struct init.
+  constexpr E(int(&)[6]) {}
+};
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init5.C gcc/testsuite/g++.dg/cpp2a/constexpr-init5.C
new file mode 100644
index 00000000000..5c2b545086c
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init5.C
@@ -0,0 +1,23 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct S { int i; };
+
+constexpr void
+fn ()
+{
+  S s;
+
+  []() constexpr {
+    int i;
+  }();
+}
+
+constexpr int
+fn2 ()
+{
+  return __extension__ ({ int n; n; });
+}
+
+constexpr int i = fn2 (); // { dg-error "not usable in a constant expression" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init6.C gcc/testsuite/g++.dg/cpp2a/constexpr-init6.C
new file mode 100644
index 00000000000..a2994f5272c
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init6.C
@@ -0,0 +1,26 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+/* We used to get the "constexpr constructor for union S::<unnamed union>
+   must initialize exactly one non-static data member" error, but not anymore
+   in C++20.  */
+
+struct S {
+  union {
+    int i;
+    double d;
+  };
+  constexpr S() { }
+};
+
+union U {
+  int a;
+  constexpr U() { }
+};
+
+struct W {
+  union {
+    int a;
+  };
+  constexpr W() { }
+};
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init7.C gcc/testsuite/g++.dg/cpp2a/constexpr-init7.C
new file mode 100644
index 00000000000..dd2741efa8c
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init7.C
@@ -0,0 +1,63 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct S {
+  int a = 1;
+  constexpr S() = default;
+};
+
+constexpr S s;
+
+union U {
+  int a = 1;
+  constexpr U() = default;
+};
+
+constexpr U u;
+
+struct S2 {
+  int a;
+  constexpr S2() = default;
+};
+
+constexpr S2 s2; // { dg-error "uninitialized .const s2." }
+
+union U2 {
+  int a;
+  constexpr U2() = default;
+};
+
+constexpr U2 u2; // { dg-error "uninitialized .const u2." }
+
+struct S3 {
+  // FIXME if it's anonymous union, we don't give the error below
+  union {
+    int a;
+  } u;
+  constexpr S3() = default;
+};
+
+constexpr S3 s3; // { dg-error "uninitialized .const s3." }
+
+struct S4 {
+  // FIXME if it's anonymous union, we don't give the error below
+  union {
+    int n;
+  } u;
+  constexpr S4() = default;
+};
+
+constexpr S4 s4; // { dg-error "uninitialized .const s4." }
+
+struct S5 {
+  union {
+    int n = 0;
+  };
+  // FIXME if it's anonymous union, we don't give the error below
+  union {
+    int m;
+  } u;
+  constexpr S5() = default;
+};
+
+constexpr S5 s5; // { dg-error "uninitialized .const s5.|not a constant expression" }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init8.C gcc/testsuite/g++.dg/cpp2a/constexpr-init8.C
new file mode 100644
index 00000000000..0d5a4a79c90
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init8.C
@@ -0,0 +1,15 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct S {
+  constexpr S(int) {}
+};
+
+struct W {
+  constexpr W(int) : s(8), p() {}
+
+  S s;
+  int *p;
+};
+
+constexpr auto a = W(42);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-init9.C gcc/testsuite/g++.dg/cpp2a/constexpr-init9.C
new file mode 100644
index 00000000000..b44098cc89b
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-init9.C
@@ -0,0 +1,17 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+
+struct S {
+  constexpr S(int) {}
+};
+
+struct W {
+  constexpr W(int (&)[8]) : W(8) { }
+  constexpr W(int) : s(8), p() {}
+
+  S s;
+  int *p;
+};
+
+int arr[8];
+constexpr auto a = W(arr);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
index 47cdce88e36..3b51bf7c901 100644
--- gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
@@ -4,13 +4,13 @@
 
 constexpr int foo ()
 try {			// { dg-warning "function-try-block body of 'constexpr' function only available with" "" { target c++17_down } }
-  int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" }
+  int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } }
   static double b = 1.0;// { dg-error "'b' declared 'static' in 'constexpr' function" }
   goto l;		// { dg-error "'goto' in 'constexpr' function" }
   l:;
   return 0;
 } catch (...) {
-  long int c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" }
+  long int c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" "" { target c++17_down } }
   static float d = 2.0f;// { dg-error "'d' declared 'static' in 'constexpr' function" }
   goto l2;		// { dg-error "'goto' in 'constexpr' function" }
   l2:;
@@ -19,19 +19,19 @@ try {			// { dg-warning "function-try-block body of 'constexpr' function only av
 
 constexpr int bar ()
 {
-  int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" }
+  int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } }
   static long double b = 3.0;// { dg-error "'b' declared 'static' in 'constexpr' function" }
   goto l;		// { dg-error "'goto' in 'constexpr' function" }
   l:;
   try {			// { dg-warning "'try' in 'constexpr' function only available with" "" { target c++17_down } }
-    short c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" }
+    short c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" "" { target c++17_down } }
     static float d;	// { dg-error "'d' declared 'static' in 'constexpr' function" }
-			// { dg-error "uninitialized variable 'd' in 'constexpr' function" "" { target *-*-* } .-1 }
+			// { dg-error "uninitialized variable 'd' in 'constexpr' function" "" { target c++17_down } .-1 }
     goto l2;		// { dg-error "'goto' in 'constexpr' function" }
     l2:;
     return 0;
   } catch (int) {
-    char e;		// { dg-error "uninitialized variable 'e' in 'constexpr' function" }
+    char e;		// { dg-error "uninitialized variable 'e' in 'constexpr' function" "" { target c++17_down } }
     static int f = 5;	// { dg-error "'f' declared 'static' in 'constexpr' function" }
     goto l3;		// { dg-error "'goto' in 'constexpr' function" }
     l3:;
diff --git gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
index 95251c2f5c6..7b96b76a3f3 100644
--- gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
+++ gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
@@ -134,8 +134,8 @@
 
 #ifndef __cpp_constexpr
 #  error "__cpp_constexpr"
-#elif __cpp_constexpr != 201603
-#  error "__cpp_constexpr != 201603"
+#elif __cpp_constexpr != 201907
+#  error "__cpp_constexpr != 201907"
 #endif
 
 #ifndef __cpp_decltype_auto
diff --git gcc/testsuite/g++.dg/cpp2a/lambda-mangle.C gcc/testsuite/g++.dg/cpp2a/lambda-mangle.C
new file mode 100644
index 00000000000..8ee9b0327a5
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/lambda-mangle.C
@@ -0,0 +1,15 @@
+// Test lambda mangling
+// { dg-do compile { target c++2a } }
+// { dg-require-weak "" }
+// { dg-options "-fno-inline" }
+
+template<typename T> struct R {
+  static int x;
+};
+// "int i;" makes the op() non-constexpr in C++17.  In C++20, it does not.
+template<typename T> int R<T>::x = []{int i; return 1;}();
+template int R<int>::x;
+// Type of lambda in intializer of R<int>::x: N1RIiE1xMUlvE_E
+// Corresponding operator(): _ZNK1RIiE1xMUlvE_clEv
+// { dg-final { scan-assembler-not "_ZNK1RIiE1xMUlvE_clEv" } }
+// { dg-final { scan-assembler-not "weak\[^\n\r\]*_?_ZNK1RIiE1xMUlvE_clEv" } }
diff --git gcc/testsuite/g++.dg/debug/dwarf2/pr44641.C gcc/testsuite/g++.dg/debug/dwarf2/pr44641.C
index d50df25f693..1139f412fda 100644
--- gcc/testsuite/g++.dg/debug/dwarf2/pr44641.C
+++ gcc/testsuite/g++.dg/debug/dwarf2/pr44641.C
@@ -1,5 +1,5 @@
 // Origin: PR 44641
-// { dg-do compile }
+// { dg-do compile { target c++17_down } }
 // { dg-options "-gdwarf-2 -O0 -dA" }
 
 template <class A> struct MisplacedDbg;
@@ -40,3 +40,11 @@ struct MisplacedDbg  // { dg-function-on-line {_ZN12MisplacedDbgI3ArgEC[12]Ev} {
 static MisplacedDbg<Arg> static_var1;
 static MisplacedDbg<Arg*> static_var2;
 static MisplacedDbg<Full> static_var3;
+
+// This test is skipped in C++20 because we consider the default constructor
+// MisplacedDbg() constexpr despite the uninitialized member "int i;".  So
+// the calls to
+//    MisplacedDbg<Arg>::MisplacedDbg()
+//    MisplacedDbg<Full>::MisplacedDbg()
+//    MisplacedDbg<Arg*>::MisplacedDbg()
+// are elided.  (This comment is here not to mess up the line numbers.)
diff --git gcc/testsuite/g++.dg/ext/stmtexpr21.C gcc/testsuite/g++.dg/ext/stmtexpr21.C
index 259cb2f1913..97052a117a7 100644
--- gcc/testsuite/g++.dg/ext/stmtexpr21.C
+++ gcc/testsuite/g++.dg/ext/stmtexpr21.C
@@ -7,7 +7,7 @@ struct test { const int *addr; };
 const test* setup()
 {
   static constexpr test atest =
-    { ({ int inner; (const int*)(0); }) };  // { dg-error "uninitialized" }
+    { ({ int inner; (const int*)(0); }) };  // { dg-error "uninitialized" "" { target c++17_down } }
 
   return &atest;
 }
