From 8663b6dcdcdccfbbed23278ea0b6b42e237892cb Mon Sep 17 00:00:00 2001
From: Abul Hossain Khan <140191921+abulgit@users.noreply.github.com>
Date: Tue, 18 Mar 2025 05:58:57 +0530
Subject: [PATCH] Fix : Improved error message on override mismatch (#21023)

---
 compiler/src/dmd/funcsem.d                 | 37 ++++++++++++++++++++++
 compiler/test/fail_compilation/test20489.d | 20 ++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 compiler/test/fail_compilation/test20489.d

diff --git a/compiler/src/dmd/funcsem.d b/compiler/src/dmd/funcsem.d
index e70cffb321..71d3dfa5f0 100644
--- a/compiler/src/dmd/funcsem.d
+++ b/compiler/src/dmd/funcsem.d
@@ -983,6 +983,43 @@ void funcDeclarationSemantic(Scope* sc, FuncDeclaration funcdecl)
 
                         error(funcdecl.loc, "function `%s` does not override any function, did you mean to override `%s`?",
                             funcdeclToChars, buf1.peekChars());
+
+                        // Supplemental error for parameter scope differences
+                        auto tf1 = cast(TypeFunction)funcdecl.type;
+                        auto tf2 = cast(TypeFunction)fd.type;
+
+                        if (tf1 && tf2)
+                        {
+                            auto params1 = tf1.parameterList;
+                            auto params2 = tf2.parameterList;
+
+                            if (params1.length == params2.length)
+                            {
+                                bool hasScopeDifference = false;
+
+                                for (size_t i = 0; i < params1.length; i++)
+                                {
+                                    auto p1 = params1[i];
+                                    auto p2 = params2[i];
+
+                                    if ((p1.storageClass & STC.scope_) == (p2.storageClass & STC.scope_))
+                                        continue;
+
+                                    if (!(p2.storageClass & STC.scope_))
+                                        continue;
+
+                                    if (!hasScopeDifference)
+                                    {
+                                        // Intended signature
+                                        errorSupplemental(funcdecl.loc, "Did you intend to override:");
+                                        errorSupplemental(funcdecl.loc, "`%s`", buf1.peekChars());
+                                        hasScopeDifference = true;
+                                    }
+                                    errorSupplemental(funcdecl.loc, "Parameter %d is missing `scope`",
+                                    cast(int)(i + 1));
+                                }
+                            }
+                        }
                    }
                 }
                 else
diff --git a/compiler/test/fail_compilation/test20489.d b/compiler/test/fail_compilation/test20489.d
new file mode 100644
index 0000000000..1cd5350959
--- /dev/null
+++ b/compiler/test/fail_compilation/test20489.d
@@ -0,0 +1,20 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/test20489.d(19): Error: function `pure nothrow @nogc @safe int test20489.D.f(int delegate(int) pure nothrow @nogc @safe body)` does not override any function, did you mean to override `pure nothrow @nogc @safe int test20489.B.f(scope int delegate(int) pure nothrow @nogc @safe)`?
+fail_compilation/test20489.d(19):        Did you intend to override:
+fail_compilation/test20489.d(19):        `pure nothrow @nogc @safe int test20489.B.f(scope int delegate(int) pure nothrow @nogc @safe)`
+fail_compilation/test20489.d(19):        Parameter 1 is missing `scope`
+---
+*/
+
+// Test case for https://github.com/dlang/dmd/issues/20489
+// Improved error message on override mismatches
+
+class B {
+    pure nothrow @nogc @safe int f(scope int delegate(int) pure nothrow @nogc @safe) { return 0; }
+}
+
+class D : B {
+    override pure nothrow @nogc @safe int f(int delegate(int) pure nothrow @nogc @safe body) { return 0; }
+}