From 011c27165edf655e8f9c2ba54064b509eefd84b5 Mon Sep 17 00:00:00 2001
From: Nick Treleaven <ntrel002@gmail.com>
Date: Fri, 18 Apr 2025 13:29:13 +0100
Subject: [PATCH] Fix wrong 'not an lvalue' error when assigning to sequence
 (#21260)

Fixes #21259.
---
 compiler/src/dmd/expressionsem.d            |  8 +++-
 compiler/test/fail_compilation/seq_assign.d | 43 +++++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)
 create mode 100644 compiler/test/fail_compilation/seq_assign.d

diff --git a/compiler/src/dmd/expressionsem.d b/compiler/src/dmd/expressionsem.d
index 8ea9262998..a638985464 100644
--- a/compiler/src/dmd/expressionsem.d
+++ b/compiler/src/dmd/expressionsem.d
@@ -10579,7 +10579,12 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             {
                 TupleDeclaration td = isAliasThisTuple(e2x);
                 if (!td)
-                    goto Lnomatch;
+                {
+                Lnomatch:
+                    error(exp.loc, "cannot assign `%s` to expression sequence `%s`",
+                        exp.e2.type.toChars(), exp.e1.type.toChars());
+                    return setError();
+                }
 
                 assert(exp.e1.type.ty == Ttuple);
                 TypeTuple tt = cast(TypeTuple)exp.e1.type;
@@ -10621,7 +10626,6 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
                 // Do not need to overwrite this.e2
                 goto Ltupleassign;
             }
-        Lnomatch:
         }
 
         /* Inside constructor, if this is the first assignment of object field,
diff --git a/compiler/test/fail_compilation/seq_assign.d b/compiler/test/fail_compilation/seq_assign.d
new file mode 100644
index 0000000000..560bc34424
--- /dev/null
+++ b/compiler/test/fail_compilation/seq_assign.d
@@ -0,0 +1,43 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/seq_assign.d(28): Error: cannot assign `int` to expression sequence `(int)`
+fail_compilation/seq_assign.d(29): Error: cannot implicitly convert expression `s` of type `string` to `int`
+fail_compilation/seq_assign.d(30): Error: cannot modify constant `2`
+fail_compilation/seq_assign.d(31): Error: mismatched sequence lengths, 1 and 2
+fail_compilation/seq_assign.d(35): Error: mismatched sequence lengths, 1 and 2
+fail_compilation/seq_assign.d(36): Error: cannot implicitly convert expression `__t_field_0` of type `string` to `int`
+fail_compilation/seq_assign.d(40): Error: cannot assign `IntString` to expression sequence `(string, int)`
+fail_compilation/seq_assign.d(42): Error: cannot implicitly convert expression `AliasSeq!(b, c)` of type `(int, int)` to `IntString`
+---
+*/
+
+alias Seq(A...) = A;
+
+struct IntString
+{
+    Seq!(int, string) expand;
+    alias this = expand;
+}
+
+void main()
+{
+    int b, c;
+    string s;
+
+    Seq!(b,) = 1; // RHS not seq
+    Seq!(b,) = Seq!(s,); // b type error
+    Seq!(2,) = Seq!(1,); // not lvalue
+    Seq!(b,) = Seq!(1, 2); // too many
+
+    auto t = Seq!("two", 3);
+    Seq!(s, b) = t; // OK
+    Seq!(b,) = t; // too many
+    Seq!(b, c) = t; // b type error
+
+    IntString t2;
+    Seq!(b, s) = t2; // OK
+    t = t2; // type mismatch
+    t2 = Seq!(b, s); // OK
+    t2 = Seq!(b, c); // c wrong type
+}