Disallow initializing structs with lists

Summary:
The following code
```
struct S {}

const S s = [1, 2, 3];
```
currently compiles in Thrift (the list content is ignored, at least in cpp2) which is obviously broken. Turn it into an error as we do for other incompatible initializers.

Add more test cases for struct initialization.

Reviewed By: avalonalex

Differential Revision: D50476253

fbshipit-source-id: 2608e3fdcf655ab8142194729e70e5590b893c22

thrift/compiler/sema/const_checker.cc

@@ -278,16 +278,7 @@ class const_checker {
       check_fields(type, value->get_map());
       return;
     }
-    // Only warn on initialization from [] for legacy reasons.
-    auto level = value->get_type() == t_const_value::CV_LIST
-        ? diagnostic_level::warning
-        : diagnostic_level::error;
-    diags_.report(
-        node_,
-        level,
-        "{} is incompatible with `{}`",
-        get_category(value),
-        type->name());
+    error("{} is incompatible with `{}`", get_category(value), type->name());
   }
 
   void check_exception(const t_exception* type, const t_const_value* value) {

thrift/compiler/test/compiler_test.cc

@@ -467,11 +467,15 @@ TEST(CompilerTest, const_double_value) {
   )");
 }
 
-TEST(CompilerTest, invalid_struct_initializer) {
+TEST(CompilerTest, struct_initializer) {
   check_compile(R"(
     struct S {}
-    const S s1 = ""; # expected-error: string is incompatible with `S`
-    const S s2 = []; # expected-warning: list is incompatible with `S`
+    const S s1 = {};   # OK
+    const S s2 = 42;   # expected-error: integer is incompatible with `S`
+    const S s3 = 4.2;  # expected-error: floating-point number is incompatible with `S`
+    const S s4 = "";   # expected-error: string is incompatible with `S`
+    const S s5 = true; # expected-error: bool is incompatible with `S`
+    const S s6 = [];   # expected-error: list is incompatible with `S`
   )");
 }