Replace the horrible method not found error message with 3 separate errors

compiler/typecheck/step1_global_symbols.jou

@@ -125,6 +125,7 @@ def typecheck_statement_with_symbol(stmt: AstStatement*) -> None:
             stmt.symbol().signature = typecheck_signature(jou_file, &stmt.method, self_class)
         case AstStatementKind.Class:
             t = create_empty_class(stmt.symbol().name)
+            t.classdata.definition = stmt
             if stmt.classdef.is_generic():
                 for nameptr = stmt.classdef.generic_typevar_names.ptr; nameptr < stmt.classdef.generic_typevar_names.end(); nameptr++:
                     t.classdata.generic_params.append(create_typevar(*nameptr))

compiler/typecheck/step3_function_and_method_bodies.jou

@@ -629,6 +629,49 @@ def nth(n: int) -> byte[100]:
     return result
 
 
+def fail_with_method_not_found(
+    state: State*,
+    location: Location,
+    self_class: Type*,
+    method_name: byte*,
+) -> noreturn:
+    assert self_class.kind == TypeKind.Class
+    class_ast = self_class.classdata.definition
+    assert class_ast != NULL
+    assert class_ast.kind == AstStatementKind.Class
+
+    msg: byte[500]
+
+    # Is the method in the class definition?
+    for stmt = class_ast.classdef.body.ptr; stmt < class_ast.classdef.body.end(); stmt++:
+        if (
+            stmt.kind == AstStatementKind.MethodDef
+            and strcmp(stmt.method.ast_signature.name, method_name) == 0
+            and strcmp(stmt.location.path, state.jou_file.path) != 0
+        ):
+            # Method exists
+            if stmt.symbol().public:
+                # TODO: generate something that can be copy/pasted to write an import statement?
+                snprintf(
+                    msg, sizeof(msg),
+                    "to use the '%s' method of class %s, you need to import the file that defines it (%s)",
+                    method_name,
+                    self_class.name,
+                    stmt.location.path,
+                )
+            else:
+                snprintf(
+                    msg, sizeof(msg),
+                    "method '%s' of class %s is private (maybe add @public to it?)",
+                    method_name,
+                    self_class.name,
+                )
+            fail(location, msg)
+
+    snprintf(msg, sizeof(msg), "class %s has no method named '%s'", self_class.name, method_name)
+    fail(location, msg)
+
+
 # This is for code that looks like obj.member(). It could be a method call or a
 # function pointer call, because `obj` may have a field whose type is funcptr.
 #
@@ -684,21 +727,7 @@ def typecheck_member_that_looks_like_method(state: State*, call: AstCall*) -> Ty
 
     field = self_class.find_class_field(member_name)
     if field == NULL:
-        # TODO: better error messages!!!!!
-        big_msg: byte* = NULL
-        asprintf(
-            &big_msg,
-            "method '%s' not found in class %s. The problem is ONE of the following: 1) you didn't import the file that defines class %.*s, 2) the '%s' method is private, 3) there really is no such method in class %.*s. I know that this is a bad compiler error message, and I will later split this into 3 different error messages.",
-            member_name,
-            self_class.name,
-            strcspn(self_class.name, "[") as int,  # List[int] --> List
-            self_class.name,
-            member_name,
-            strcspn(self_class.name, "[") as int,  # List[int] --> List
-            self_class.name,
-        )
-        assert big_msg != NULL
-        fail(func_location, big_msg)
+        fail_with_method_not_found(state, func_location, self_class, member_name)
 
     # It is a function pointer call where the function is in a class field.
     # It looks like obj.field().

compiler/types.jou

@@ -21,6 +21,7 @@ class ClassField:
 class ClassData:
     generic_params: List[Type*]   # this is the "int" part of List[int]
     fields: List[ClassField]
+    definition: AstStatement*
 
     @public
     def is_generic(self) -> bool:

tests/404/import_private_method.jou

@@ -3,4 +3,4 @@ import "./imported/privates.jou"
 def foo() -> None:
     p = PublicClass{}
     # TODO: ideally error message would say that it exists but is private
-    p.private_method()  # Error: method 'private_method' not found in class PublicClass. The problem is ONE of the following: 1) you didn't import the file that defines class PublicClass, 2) the 'private_method' method is private, 3) there really is no such method in class PublicClass. I know that this is a bad compiler error message, and I will later split this into 3 different error messages.
+    p.private_method()  # Error: method 'private_method' of class PublicClass is private (maybe add @public to it?)

tests/404/indirect_import_method.jou

@@ -2,5 +2,4 @@ import "./imported/point_factory.jou"
 
 def blah() -> int:
     p = make_point()
-    # TODO: Improve this error!
-    return p.get_sum()  # Error: method 'get_sum' not found in class Point. The problem is ONE of the following: 1) you didn't import the file that defines class Point, 2) the 'get_sum' method is private, 3) there really is no such method in class Point. I know that this is a bad compiler error message, and I will later split this into 3 different error messages.
+    return p.get_sum()  # Error: to use the 'get_sum' method of class Point, you need to import the file that defines it (tests/404/imported/point.jou)

tests/404/method.jou

@@ -3,4 +3,4 @@ class Foo:
 
 def main() -> int:
     f = Foo{x=1}
-    f.bar()  # Error: method 'bar' not found in class Foo. The problem is ONE of the following: 1) you didn't import the file that defines class Foo, 2) the 'bar' method is private, 3) there really is no such method in class Foo. I know that this is a bad compiler error message, and I will later split this into 3 different error messages.
+    f.bar()  # Error: class Foo has no method named 'bar'