c++: Don't synthesize sfk_comparison method multiple times [PR94907]

On the following testcase we ICE, because synthesize_method is called twice
on the same sfk_comparison method fndecl, the first time it works fine
because start_preparsed_function in that case sets both
current_function_decl and cfun, but second time it is called it only sets
the former and keeps cfun NULL, so we ICE when trying to store
current_function_returns_value.
I think it is just wrong to call synthesize_method multiple times, and most
synthesize_method callers avoid that by not calling it if DECL_INITIAL is
already set, so this patch does that too.

2020-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94907
	* method.c (defaulted_late_check): Don't call synthesize_method
	on constexpr sfk_comparison if it has been called on it already.

	* g++.dg/cpp2a/spaceship-synth8.C: New test.

gcc/cp/ChangeLog

+2020-05-06  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94907
+	* method.c (defaulted_late_check): Don't call synthesize_method
+	on constexpr sfk_comparison if it has been called on it already.
+
 2020-05-07  Release Manager
 
 	* GCC 10.1.0 released.

gcc/cp/method.c

@@ -2939,7 +2939,7 @@ defaulted_late_check (tree fn)
     {
       /* If the function was declared constexpr, check that the definition
 	 qualifies.  Otherwise we can define the function lazily.  */
-      if (DECL_DECLARED_CONSTEXPR_P (fn))
+      if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
 	synthesize_method (fn);
       return;
     }

gcc/testsuite/ChangeLog

 2020-05-06  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/94907
+	* g++.dg/cpp2a/spaceship-synth8.C: New test.
+
+2020-05-06  Jakub Jelinek  <jakub@redhat.com>
+
 	PR rtl-optimization/94873
 	* gcc.dg/pr94873.c: New test.
 

gcc/testsuite/g++.dg/cpp2a/spaceship-synth8.C

+// PR c++/94907
+// { dg-do compile { target c++2a } }
+
+namespace std { struct strong_ordering { }; }
+
+struct E;
+struct D {
+  virtual std::strong_ordering operator<=>(const struct E&) const = 0;
+};
+struct E : D {
+  std::strong_ordering operator<=>(const E&) const override = default;
+};