diff --git a/src/dmd/declaration.d b/src/dmd/declaration.d
index ca3378d1e641..1c8e00cbd976 100644
--- a/src/dmd/declaration.d
+++ b/src/dmd/declaration.d
@@ -315,6 +315,20 @@ extern (C++) abstract class Declaration : Dsymbol
             }
         }
 
+        if (e1 && e1.op == TOKthis && isField())
+        {
+            VarDeclaration vthis = (cast(ThisExp)e1).var;
+            for (Scope* scx = sc; scx; scx = scx.enclosing)
+            {
+                if (scx.func == vthis.parent && (scx.flags & SCOPEcontract))
+                {
+                    if (!flag)
+                        error(loc, "cannot modify parameter 'this' in contract");
+                    return 2; // do not report type related errors
+                }
+            }
+        }
+
         if (v && (isCtorinit() || isField()))
         {
             // It's only modifiable if inside the right constructor
diff --git a/src/dmd/semantic3.d b/src/dmd/semantic3.d
index f528623354d0..1632a07f6a17 100644
--- a/src/dmd/semantic3.d
+++ b/src/dmd/semantic3.d
@@ -885,7 +885,6 @@ private extern(C++) final class Semantic3Visitor : Visitor
                 sc2.flags = (sc2.flags & ~SCOPEcontract) | SCOPErequire;
 
                 // BUG: need to error if accessing out parameters
-                // BUG: need to treat parameters as const
                 // BUG: need to disallow returns and throws
                 // BUG: verify that all in and ref parameters are read
                 freq = freq.statementSemantic(sc2);
@@ -908,7 +907,6 @@ private extern(C++) final class Semantic3Visitor : Visitor
                 sc2 = scout; //push
                 sc2.flags = (sc2.flags & ~SCOPEcontract) | SCOPEensure;
 
-                // BUG: need to treat parameters as const
                 // BUG: need to disallow returns and throws
 
                 if (funcdecl.fensure && f.next.ty != Tvoid)
diff --git a/test/fail_compilation/fail18143.d b/test/fail_compilation/fail18143.d
new file mode 100644
index 000000000000..2d0f0018afa1
--- /dev/null
+++ b/test/fail_compilation/fail18143.d
@@ -0,0 +1,43 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail18143.d(20): Error: variable fail18143.S.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(21): Error: variable fail18143.S.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(25): Error: variable fail18143.S.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(26): Error: variable fail18143.S.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(35): Error: variable fail18143.C.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(36): Error: variable fail18143.C.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(40): Error: variable fail18143.C.a cannot modify parameter 'this' in contract
+fail_compilation/fail18143.d(41): Error: variable fail18143.C.a cannot modify parameter 'this' in contract
+---
+*/
+
+struct S
+{
+    int a;
+
+    this(int n)
+    in { a = n; }   // error, modifying this.a in contract
+    out { a = n; }  // error, modifying this.a in contract
+    do { }
+
+    void foo(int n)
+    in { a = n; }   // error, modifying this.a in contract
+    out { a = n; }  // error, modifying this.a in contract
+    do { }
+}
+
+class C
+{
+    int a;
+
+    this(int n)
+    in { a = n; }   // error, modifying this.a in contract
+    out { a = n; }  // error, modifying this.a in contract
+    do { }
+
+    void foo(int n)
+    in { a = n; }   // error, modifying this.a in contract
+    out { a = n; }  // error, modifying this.a in contract
+    do { }
+}