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.

2 files changed, 120 insertions, 0 deletions

diff --git a/tests/language-feature/interfaces/constructor-noncopyable-return.slang b/tests/language-feature/interfaces/constructor-noncopyable-return.slang
new file mode 100644
index 000000000..9f6c7355a
--- /dev/null
+++ b/tests/language-feature/interfaces/constructor-noncopyable-return.slang
@@ -0,0 +1,60 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+// Note: This test won't produce expected result on vulkan.
+//DISABLED_TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Executable test to check the correctness of return values from the constructors within a
+// NonCopyableType struct.
+
+// A constructor from the callsite's point of view is a function that returns the struct type.
+// A constructor from the inside the body is treated as a function that modifies `this` and not return.
+
+// In the following test-cases where the constructor either returns an expr or no value,
+// such as `return ...` within a condition for early exit or not,
+// It automatically gets resolved as
+// `return this` if it returns the struct type, else `return`
+RWStructuredBuffer<float> outputBuffer;
+
+[__NonCopyableType] struct Impl
+{
+    float x;
+    __init(float v)
+    {
+        if (v > 0)
+            {
+            this.x = 2 * v;
+            return this;
+        }
+        else
+        {
+            this.x = 3 * v;
+            return;
+        }
+    }
+
+    __init(int ival)
+    {
+        float val = ival;
+        Impl v = Impl(val);
+        return v;
+    }
+
+    __init()
+    {
+        float val = 2.0;
+        return Impl(val);
+    }
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    // CHECK: 4.000000
+    // CHECK: 2.000000
+    // CHECK: -3.000000
+    // CHECK: -6.000000
+    outputBuffer[id] = Impl().x;
+    outputBuffer[id + 1] = Impl(1).x;
+    outputBuffer[id + 2] = Impl(-1).x;
+    outputBuffer[id + 3] = Impl(-2.0).x;
+}
diff --git a/tests/language-feature/interfaces/constructor-return.slang b/tests/language-feature/interfaces/constructor-return.slang
new file mode 100644
index 000000000..85f474c57
--- /dev/null
+++ b/tests/language-feature/interfaces/constructor-return.slang
@@ -0,0 +1,60 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Executable test to check the correctness of return values from the constructors within an
+// ordinary struct.
+
+// A constructor from the callsite's point of view is a function that returns the struct type.
+// A constructor from the inside the body is treated as a function that modifies `this` and not return.
+
+// In the following test-cases where the constructor either returns an expr or no value,
+// such as `return ...` within a condition for early exit or not,
+// It automatically gets resolved as
+// `this = t;` along with
+// `return this` if it returns the struct type, else `return`
+RWStructuredBuffer<float> outputBuffer;
+
+struct Impl
+{
+    float x;
+    __init(float v)
+    {
+        if (v > 0)
+        {
+            this.x = 2 * v;
+            return this;
+        }
+        else
+        {
+            this.x = 3 * v;
+            return;
+        }
+    }
+
+    __init(int ival)
+    {
+        float val = ival;
+        Impl v = Impl(val);
+        return v;
+    }
+
+    __init()
+    {
+        float val = 2.0;
+        return Impl(val);
+    }
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    // CHECK: 4.000000
+    // CHECK: 2.000000
+    // CHECK: -3.000000
+    // CHECK: -6.000000
+    outputBuffer[id] = Impl().x;
+    outputBuffer[id + 1] = Impl(1).x;
+    outputBuffer[id + 2] = Impl(-1).x;
+    outputBuffer[id + 3] = Impl(-2.0).x;
+}