Fix __init() functions that returns an existing value (#3866)

Fixes the issue #3671

* The __init constructors are not expected to return a value like other member
  functions, but must construct a new value and return the struct type or none.

* This patch enables this behavior in the IR lowering without complaining about
  illegal situations where the user returns an invalid type or none at all.
   Translate    ordinary struct `return ...;` to `this = ...; return this;`
   Translate NonCopyableType struct `return ...;` to `return this;`

* This patch also fixes the issue with type checking when __init()
  returns a void that mismatches the base type of the struct/ class
   Translate    ordinary struct `return;` to `return this;`
   Translate NonCopyableType struct `return;` to `return;`

* Add end-to-end test and compile only tests to check the above behavior.

1 files changed, 1 insertions, 1 deletions

diff --git a/source/slang/slang-check-stmt.cpp b/source/slang/slang-check-stmt.cpp
index 729b24d35..f7b085579 100644
--- a/source/slang/slang-check-stmt.cpp
+++ b/source/slang/slang-check-stmt.cpp
@@ -392,7 +392,7 @@ namespace Slang
         auto function = getParentFunc();
         if (!stmt->expression)
         {
-            if (function && !function->returnType.equals(m_astBuilder->getVoidType()))
+            if (function && !function->returnType.equals(m_astBuilder->getVoidType()) && !as<ConstructorDecl>(function))
             {
                 getSink()->diagnose(stmt, Diagnostics::returnNeedsExpression);
             }