From 5fa2707760587d12e203e526ea919f45f5cd1bac Mon Sep 17 00:00:00 2001
From: Herb Sutter <herb.sutter@gmail.com>
Date: Sun, 9 Apr 2023 12:20:06 -0700
Subject: [PATCH] Add diagnostics for `sometype.` and `object::`, closes #339

---
 source/cppfront.cpp | 42 ++++++++++++++++++++++++++++++++++++++++++
 source/parse.h      | 10 ++++++++++
 source/sema.h       | 12 ++++++++++++
 3 files changed, 64 insertions(+)

diff --git a/source/cppfront.cpp b/source/cppfront.cpp
index f17ddc033..fb742e41a 100644
--- a/source/cppfront.cpp
+++ b/source/cppfront.cpp
@@ -1626,6 +1626,27 @@ class cppfront
     auto emit(qualified_id_node const& n)
         -> void
     {
+        //  Check for some incorrect uses of :: or .
+        if (auto decl = sema.get_declaration_of(n.get_first_token(), true);
+            decl && std::ssize(n.ids) > 1
+            )
+        {
+            assert (decl->declaration);
+
+            if (
+                decl->declaration->is_object()
+                && n.ids[1].scope_op
+                && n.ids[1].scope_op->type() == lexeme::Scope
+                )
+            {
+                errors.emplace_back(
+                    n.position(),
+                    "use '" + decl->identifier->to_string(true) + ".' to refer to an object member"
+                );
+                return;
+            }
+        }
+
         //  Implicit "cpp2::" qualification of "unique.new" and "shared.new"
         if (
             n.ids.size() == 2
@@ -2536,6 +2557,27 @@ class cppfront
         assert(n.expr);
         last_postfix_expr_was_pointer = false;
 
+        //  Check for some incorrect uses of :: or .
+        if (auto decl = sema.get_declaration_of(n.get_first_token_ignoring_this(), true);
+            decl && !n.ops.empty()
+            )
+        {
+            assert (decl->declaration);
+
+            if (
+                decl->declaration->is_type()
+                && n.ops[0].op
+                && n.ops[0].op->type() == lexeme::Dot
+                )
+            {
+                errors.emplace_back(
+                    n.position(),
+                    "use '" + decl->identifier->to_string(true) + "::' to refer to a type member"
+                );
+                return;
+            }
+        }
+
         //  For a 'move that' parameter, track the members we already moved from
         //  so we can diagnose attempts to move from the same member twice
         if (
diff --git a/source/parse.h b/source/parse.h
index c08efb21f..d13f7a1e1 100644
--- a/source/parse.h
+++ b/source/parse.h
@@ -636,6 +636,16 @@ struct qualified_id_node
         return {};
     }
 
+    auto get_first_token() const
+        -> token const*
+    {
+        assert (
+            !ids.empty()
+            && ids.front().id
+        );
+        return ids.front().id->get_token();
+    }
+
     auto position() const
         -> source_position
     {
diff --git a/source/sema.h b/source/sema.h
index 65128d04b..31666e444 100644
--- a/source/sema.h
+++ b/source/sema.h
@@ -263,6 +263,18 @@ class sema
 
     //  Get the declaration of t within the same named function or beyond it
     //
+    auto get_declaration_of(
+        token const* t,
+        bool         look_beyond_current_function = false
+    )
+        -> declaration_sym const*
+    {
+        if (!t) {
+            return {};
+        }
+        return get_declaration_of(*t, look_beyond_current_function);
+    }
+
     auto get_declaration_of(
         token const& t,
         bool         look_beyond_current_function = false