From a80d302a2f226f4ba0c45ed7b0a214aa2095f51f Mon Sep 17 00:00:00 2001
From: Nick Treleaven <ntrel002@gmail.com>
Date: Sat, 7 Dec 2024 13:19:33 +0000
Subject: [PATCH] Don't allow short constructor with non-void expression

Follow up to #17079.
---
 compiler/src/dmd/dsymbolsem.d                | 18 ++++++++++++++++++
 compiler/test/compilable/shortened_methods.d |  2 +-
 compiler/test/fail_compilation/short_fn.d    | 15 +++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 compiler/test/fail_compilation/short_fn.d

diff --git a/compiler/src/dmd/dsymbolsem.d b/compiler/src/dmd/dsymbolsem.d
index 173532af397c..65aab8a6dedc 100644
--- a/compiler/src/dmd/dsymbolsem.d
+++ b/compiler/src/dmd/dsymbolsem.d
@@ -2432,6 +2432,24 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
         sc.stc &= ~STC.static_; // not a static constructor
 
         funcDeclarationSemantic(sc, ctd);
+        // Check short constructor: this() => expr;
+        if (ctd.fbody)
+        {
+            if (auto s = ctd.fbody.isExpStatement())
+            {
+                if (s.exp)
+                {
+                    auto ce = s.exp.isCallExp();
+                    // check this/super before semantic
+                    if (!ce || (!ce.e1.isThisExp() && !ce.e1.isSuperExp()))
+                    {
+                        s.exp = s.exp.expressionSemantic(sc);
+                        if (s.exp.type.ty != Tvoid)
+                            error(s.loc, "can only return void expression, `this` call or `super` call from constructor");
+                    }
+                }
+            }
+        }
 
         sc.pop();
 
diff --git a/compiler/test/compilable/shortened_methods.d b/compiler/test/compilable/shortened_methods.d
index 81d6da8734e1..76a4c8a55db6 100644
--- a/compiler/test/compilable/shortened_methods.d
+++ b/compiler/test/compilable/shortened_methods.d
@@ -15,7 +15,7 @@ class A {
     bool isNull() => this is null;
 
     this() {}
-    this(int x) => _x = x;
+    this(int x) { _x = x; }
     this(float y) => this(cast(int) y);
 }
 
diff --git a/compiler/test/fail_compilation/short_fn.d b/compiler/test/fail_compilation/short_fn.d
new file mode 100644
index 000000000000..04f3e3fffbfa
--- /dev/null
+++ b/compiler/test/fail_compilation/short_fn.d
@@ -0,0 +1,15 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/short_fn.d(13): Error: can only return void expression, `this` call or `super` call from constructor
+fail_compilation/short_fn.d(14): Error: can only return void expression, `this` call or `super` call from constructor
+---
+*/
+
+struct Number
+{
+    int x;
+
+    this(int x) => this.x = x;
+    this(byte x) => Number();
+}