Fix Issue 18743 - ConditionalExpression and AssignExpression should r…

…equire parens

src/dmd/parse.d

@@ -8489,6 +8489,11 @@ final class Parser(AST) : Lexer
     AST.Expression parseAssignExp()
     {
         auto e = parseCondExp();
+            // require parens for e.g. a ? a : a = a;
+            if (precedence[e.op] == PREC.cond && !e.parens && precedence[token.value] == PREC.assign)
+                dmd.errors.deprecation(e.loc, "`%s` must be parenthesized when next to operator `%s`",
+                    e.toChars(), Token.toChars(token.value));
+
         const loc = token.loc;
         switch (token.value)
         {

test/fail_compilation/bug18743.d

@@ -0,0 +1,22 @@
+// REQUIRED_ARGS: -de
+/*
+TEST_OUTPUT:
+---
+fail_compilation/bug18743.d(18): Deprecation: `a ? a = 4 : a` must be parenthesized when next to operator `=`
+fail_compilation/bug18743.d(19): Deprecation: `a ? --a : a` must be parenthesized when next to operator `+=`
+---
+*/
+
+void main()
+{
+	int a;
+
+	// ok
+	(a ? a = 4 : a) = 5;
+	a ? a = 4 : (a = 5);
+
+	a ? a = 4 : a = 5;
+	a ? --a : a += 1;
+
+	a ? a = 4 : a++; // ok
+}