Disallow initializing fields with themself

changelog/dmd.deprecation-noop-assignment.dd

@@ -0,0 +1,15 @@
+Initializing a field with itself has been deprecated
+
+This is to prevent a common mistake when typing a simple constructor, where a parameter name is misspelled:
+
+---
+struct S
+{
+    int field;
+
+    this(int feild)
+    {
+        this.field = field; // equal to this.field = this.field
+    }
+}
+---

compiler/src/dmd/expressionsem.d

@@ -10604,6 +10604,35 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
         if (exp.op == EXP.assign
             && exp.e1.checkModifiable(sc) == Modifiable.initialization)
         {
+            // Check common mistake of misspelled parameters in constructors,
+            // e.g. `this(int feild) { this.field = field; }`
+            if (auto dve1 = exp.e1.isDotVarExp)
+                if (auto dve2 = exp.e2.isDotVarExp)
+                    if (sc.func && sc.func.parameters && dve1.e1.isThisExp && dve2.e1.isThisExp()
+                        && dve1.var.ident.equals(dve2.var.ident))
+                    {
+                        // @@@DEPRECATED_2.121@@@
+                        // Deprecated in 2.111, make it an error in 2.121
+                        deprecation(exp.e1.loc, "cannot initialize field `%s` with itself", dve1.var.toChars());
+                        auto findParameter(const(char)[] s, ref int cost)
+                        {
+                            foreach (p; *sc.func.parameters)
+                            {
+                                if (p.ident.toString == s)
+                                {
+                                    cost = 1;
+                                    return p.ident.toString;
+                                }
+                            }
+                            return null;
+                        }
+                        import dmd.root.speller : speller;
+                        if (auto s = speller!findParameter(dve1.var.ident.toString))
+                        {
+                            deprecationSupplemental(sc.func.loc, "did you mean to use parameter `%.*s`?\n", s.fTuple.expand);
+                        }
+                    }
+
             //printf("[%s] change to init - %s\n", exp.loc.toChars(), exp.toChars());
             auto t = exp.type;
             exp = new ConstructExp(exp.loc, exp.e1, exp.e2);

compiler/test/compilable/interpret3.d

@@ -6240,9 +6240,9 @@ struct Coord13831
 
 struct Chunk13831
 {
-    this(Coord13831)
+    this(Coord13831 coord)
     {
-        coord = coord;
+        this.coord = coord;
     }
 
     Coord13831 coord;

compiler/test/compilable/test22510.d

@@ -7,7 +7,7 @@ struct S
     @disable this(this);
     this (scope ref inout S) inout
     {
-    	this.b = b;
+    	this.b = 0;
     }
 }
 

compiler/test/fail_compilation/ctor_self_assignment.d

@@ -0,0 +1,23 @@
+/**
+REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/ctor_self_assignment.d(17): Deprecation: cannot initialize field `location` with itself
+fail_compilation/ctor_self_assignment.d(15):        did you mean to use parameter `locaction`?
+---
+*/
+// https://forum.dlang.org/post/[email protected]
+
+alias Location = int;
+
+struct Node
+{
+    this(Location locaction, uint f)
+    {
+        this.location = location;
+        this.f = f;
+    }
+
+    Location location;
+    uint f;
+}