Add assertion if nullptr is passed to parse function (#3593)

Addresses #3584

docs/mkdocs/docs/api/basic_json/accept.md

@@ -29,7 +29,7 @@ Unlike the [`parse`](parse.md) function, this function neither throws an excepti
 :   A compatible input, for instance:
     
     - an `std::istream` object
-    - a `FILE` pointer
+    - a `FILE` pointer (must not be null)
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
     - a `std::string`
@@ -72,6 +72,11 @@ Linear in the length of the input. The parser is a predictive LL(1) parser.
 
 (1) A UTF-8 byte order mark is silently ignored.
 
+!!! danger "Runtime assertion"
+
+    The precondition that a passed `#!cpp FILE` pointer must not be null is enforced with a
+    [runtime assertion](../../features/assertions.md).
+
 ## Examples
 
 ??? example

docs/mkdocs/docs/api/basic_json/parse.md

@@ -28,7 +28,7 @@ static basic_json parse(IteratorType first, IteratorType last,
 :   A compatible input, for instance:
     
     - an `std::istream` object
-    - a `FILE` pointer
+    - a `FILE` pointer (must not be null)
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
     - a `std::string`
@@ -71,6 +71,13 @@ Deserialized JSON value; in case of a parse error and `allow_exceptions` set to
 
 Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
 
+## Exceptions
+
+- Throws [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
+- Throws [`parse_error.102`](../../home/exceptions.md#jsonexceptionparse_error102) if to_unicode fails or surrogate
+  error.
+- Throws [`parse_error.103`](../../home/exceptions.md#jsonexceptionparse_error103) if to_unicode fails.
+
 ## Complexity
 
 Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser
@@ -81,6 +88,11 @@ super-linear complexity.
 
 (1) A UTF-8 byte order mark is silently ignored.
 
+!!! danger "Runtime assertion"
+
+    The precondition that a passed `#!cpp FILE` pointer must not be null is enforced with a
+    [runtime assertion](../../features/assertions.md).
+
 ## Examples
 
 ??? example "Parsing from a character array"

docs/mkdocs/docs/features/assertions.md

@@ -102,3 +102,30 @@ behavior and yields a runtime assertion.
     ```
     Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
     ```
+
+### Reading from a null `FILE` pointer
+
+Reading from a null `#!cpp FILE` pointer is undefined behavior and yields a runtime assertion. This can happen when
+calling `#!cpp std::fopen` on a nonexistent file.
+
+??? example "Example 4: Uninitialized iterator"
+
+    The following code will trigger an assertion at runtime:
+
+    ```cpp
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+      std::FILE* f = std::fopen("nonexistent_file.json", "r");
+      json j = json::parse(f);
+    }
+    ```
+
+    Output:
+
+    ```
+    Assertion failed: (m_file != nullptr), function file_input_adapter, file input_adapters.hpp, line 55.
+    ```

include/nlohmann/detail/input/input_adapters.hpp

@@ -50,7 +50,9 @@ class file_input_adapter
     JSON_HEDLEY_NON_NULL(2)
     explicit file_input_adapter(std::FILE* f) noexcept
         : m_file(f)
-    {}
+    {
+        JSON_ASSERT(m_file != nullptr);
+    }
 
     // make class move-only
     file_input_adapter(const file_input_adapter&) = delete;

single_include/nlohmann/json.hpp

@@ -5936,7 +5936,9 @@ class file_input_adapter
     JSON_HEDLEY_NON_NULL(2)
     explicit file_input_adapter(std::FILE* f) noexcept
         : m_file(f)
-    {}
+    {
+        JSON_ASSERT(m_file != nullptr);
+    }
 
     // make class move-only
     file_input_adapter(const file_input_adapter&) = delete;