c++: Fix poor diagnostic for array initializer [PR93710]

A small improvement for an error in build_user_type_conversion_1:
instead of

array-init1.C:11:1: error: conversion from ‘long int’ to ‘A’ is ambiguous
   11 | };
      | ^

we will print

array-init1.C:8:3: error: conversion from ‘long int’ to ‘A’ is ambiguous
    8 |   0L,
      |   ^~

2020-02-12  Marek Polacek  <polacek@redhat.com>

	PR c++/93710 - poor diagnostic for array initializer.
	* call.c (build_user_type_conversion_1): Use cp_expr_loc_or_input_loc
	for an error call.

	* g++.dg/diagnostic/array-init1.C: New test.

gcc/cp/ChangeLog

+2020-02-15  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93710 - poor diagnostic for array initializer.
+	* call.c (build_user_type_conversion_1): Use cp_expr_loc_or_input_loc
+	for an error call.
+
 2020-02-15  Jason Merrill  <jason@redhat.com>
 
 	PR c++/92556

gcc/cp/call.c

@@ -4172,8 +4172,9 @@ build_user_type_conversion_1 (tree totype, tree expr, int flags,
       if (complain & tf_error)
 	{
 	  auto_diagnostic_group d;
-	  error ("conversion from %qH to %qI is ambiguous",
-		 fromtype, totype);
+	  error_at (cp_expr_loc_or_input_loc (expr),
+		    "conversion from %qH to %qI is ambiguous",
+		    fromtype, totype);
 	  print_z_candidates (location_of (expr), candidates);
 	}
 

gcc/testsuite/ChangeLog

+2020-02-15  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93710 - poor diagnostic for array initializer.
+	* g++.dg/diagnostic/array-init1.C: New test.
+
 2020-02-15  Jason Merrill  <jason@redhat.com>
 
 	* lib/target-supports.exp (check_effective_target_c++2a_only): Also

gcc/testsuite/g++.dg/diagnostic/array-init1.C

+// PR c++/93710 - poor diagnostic for array initializer.
+
+struct A { A (int); A (char*); int i; };
+
+int x;
+
+A a1[] = {
+  0L, // { dg-error "3:conversion from .long int. to .A. is ambiguous" }
+  &x, // { dg-error "3:invalid conversion from .int\\*. to .int." }
+  __builtin_offsetof (A, i) // { dg-error "23:conversion from .long unsigned int. to .A. is ambiguous" }
+};