Disallow assigning to struct rvalue

compiler/src/dmd/expressionsem.d

@@ -10747,6 +10747,13 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
         // printf("PreExp::semantic('%s')\n", toChars());
         if (Expression e = exp.opOverloadUnary(sc))
         {
+            if (!sc.intypeof && exp.e1.type && exp.e1.type.ty == Tstruct &&
+                !exp.e1.isLvalue())
+            {
+                error(exp.e1.loc, "cannot modify struct rvalue `%s`",
+                    exp.e1.toChars());
+                e = ErrorExp.get();
+            }
             result = e;
             return;
         }

compiler/src/dmd/opover.d

@@ -537,8 +537,15 @@ Expression opOverloadAssign(AssignExp e, Scope* sc, Type[2] aliasThisStop)
 
     bool choseReverse;
     if (auto result = pickBestBinaryOverload(sc, null, s, null, e, choseReverse))
+    {
+        if (!sc.intypeof && e.e1.type.ty == Tstruct && !e.e1.isLvalue())
+        {
+            error(e.e1.loc, "cannot assign to struct rvalue `%s`",
+                e.e1.toChars());
+            return ErrorExp.get();
+        }
         return result;
-
+    }
     return binAliasThis(e, sc, aliasThisStop);
 }
 
@@ -1004,6 +1011,11 @@ Expression opOverloadBinaryAssign(BinAssignExp e, Scope* sc, Type[2] aliasThisSt
     if (e.e1.type.isTypeError() || e.e2.type.isTypeError())
         return ErrorExp.get();
 
+    if (!sc.intypeof && e.e1.type.ty == Tstruct && !e.e1.isLvalue())
+    {
+        error(e.e1.loc, "cannot assign to struct rvalue `%s`", e.e1.toChars());
+        return ErrorExp.get();
+    }
     AggregateDeclaration ad1 = isAggregate(e.e1.type);
     Dsymbol s = search_function(ad1, Id.opOpAssign);
     if (s && !(s.isTemplateDeclaration() || s.isOverloadSet()))

compiler/test/fail_compilation/fail9936.d

@@ -3,7 +3,7 @@ TEST_OUTPUT:
 ---
 fail_compilation/fail9936.d(25): Error: `S().opBinary` isn't a template
 fail_compilation/fail9936.d(26): Error: `S().opBinaryRight` isn't a template
-fail_compilation/fail9936.d(27): Error: `S().opOpAssign` isn't a template
+fail_compilation/fail9936.d(27): Error: `s.opOpAssign` isn't a template
 fail_compilation/fail9936.d(29): Error: `S().opIndexUnary` isn't a template
 fail_compilation/fail9936.d(30): Error: `S().opUnary` isn't a template
 ---
@@ -17,14 +17,14 @@ struct S
     auto opIndexUnary(S s) { return 1; }
     auto opUnary(S s) { return 1; }
 }
-void main()
+void f(S s)
 {
     static assert(!is(typeof( S() + S() )));
     static assert(!is(typeof( 100 + S() )));
-    static assert(!is(typeof( S() += S() )));
+    static assert(!is(typeof( s += S() )));
     S() + S();
     100 + S();
-    S() += S();
+    s += S();
 
     +S()[0];
     +S();

compiler/test/fail_compilation/struct_rvalue_assign.d

@@ -0,0 +1,33 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/struct_rvalue_assign.d(12): Error: cannot assign to struct rvalue `foo()`
+fail_compilation/struct_rvalue_assign.d(13): Error: cannot assign to struct rvalue `foo()`
+fail_compilation/struct_rvalue_assign.d(14): Error: cannot modify struct rvalue `foo()`
+---
+*/
+
+void main ()
+{
+    foo() = S.init;
+    foo() += 5;
+    ++foo();
+}
+
+S foo()
+{
+    return S.init;
+}
+
+struct S
+{
+    int i;
+
+    void opAssign(S s)
+    {
+        this.i = s.i;
+    }
+
+    void opOpAssign(string op : "+")(int) {}
+    void opUnary(string op : "++")() {}
+}