Allow shortened method syntax for constructors

changelog/dmd.shortened-method-constructor.dd

@@ -0,0 +1,15 @@
+Shortened method syntax can now be used in constructors
+
+This used to raise an error "cannot return expression from constructor", but it's now supported:
+
+---
+struct Number
+{
+    int x;
+
+    this(int x) => this.x = x;
+    this(float x) => this(cast(int) x);
+}
+---
+
+Postblits and destructors already supported shortened method syntax because they return `void`.

compiler/src/dmd/parse.d

@@ -5252,7 +5252,10 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
                 error("missing `do { ... }` after `in` or `out`");
             const returnloc = token.loc;
             nextToken();
-            f.fbody = new AST.ReturnStatement(returnloc, parseExpression());
+            if (f.isCtorDeclaration)
+                f.fbody = new AST.ExpStatement(returnloc, parseExpression());
+            else
+                f.fbody = new AST.ReturnStatement(returnloc, parseExpression());
             f.endloc = token.loc;
             check(TOK.semicolon);
             break;

compiler/test/compilable/shortened_methods.d

@@ -13,6 +13,10 @@ class A {
 
     // or normal method defintions
     bool isNull() => this is null;
+
+    this() {}
+    this(int x) => _x = x;
+    this(float y) => this(cast(int) y);
 }
 
 class B : A{
@@ -36,3 +40,12 @@ void func() {
     // Issue 24088 - https://issues.dlang.org/show_bug.cgi?id=24088
     S!int f() => S!int();
 }
+
+struct T
+{
+    void inc() {}
+    this(this) => inc();
+
+    void free() {}
+    ~this() => free();
+}