Fix a bug in struct inheritance (#1861)

During lowering from AST to IR, the Slang compiler translates code that uses `struct` inheritance:

```hlsl
struct Base { int a; }
struct Derived : Base {}
```

into code where the inheritance relationship is "witnessed" by a simple field:

```hlsl
struct Base { int a; }
struct Derived { Base __anonymous_field__; }
```

The underlying bug here is that the `__anonymous_field__` that the compiler generated during IR lowering was not being given any linkage decorations (no mangled name). As a result, if multiple separately-compiled modules all access that field they could disagree on its identity as an IR instruction. This could lead to output code being generated where the declaration of `__anonymous_field__` uses one IR instruction, but accesses use another.

This change includes a fix for the issue, and a test that serves as a reproducer for the original problem.

3 files changed, 44 insertions, 0 deletions

diff --git a/tests/language-feature/inheritance/struct-inheritance-import.slang b/tests/language-feature/inheritance/struct-inheritance-import.slang
new file mode 100644
index 000000000..0ea49957e
--- /dev/null
+++ b/tests/language-feature/inheritance/struct-inheritance-import.slang
@@ -0,0 +1,28 @@
+// struct-inheritance-import.slang
+
+//TEST(compute):COMPARE_COMPUTE:
+
+// This test confirms that the synthesized code for struct type inheritance
+// includes proper linkage decorations so that it can work across modules.
+
+import struct_inheritance_imported;
+
+int test(int val)
+{
+    Derived d;
+    d.a = val;
+
+    return 0x100 + d.a*0x10 + getA(d);
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    uint tid = dispatchThreadID.x;
+    int inVal = tid;
+    int outVal = test(inVal);
+    outputBuffer[tid] = outVal;
+}
diff --git a/tests/language-feature/inheritance/struct-inheritance-import.slang.expected.txt b/tests/language-feature/inheritance/struct-inheritance-import.slang.expected.txt
new file mode 100644
index 000000000..60d3d339d
--- /dev/null
+++ b/tests/language-feature/inheritance/struct-inheritance-import.slang.expected.txt
@@ -0,0 +1,4 @@
+100
+111
+122
+133
diff --git a/tests/language-feature/inheritance/struct-inheritance-imported.slang b/tests/language-feature/inheritance/struct-inheritance-imported.slang
new file mode 100644
index 000000000..8786b851c
--- /dev/null
+++ b/tests/language-feature/inheritance/struct-inheritance-imported.slang
@@ -0,0 +1,12 @@
+//TEST_IGNORE_FILE:
+// struct-inheritance-imported.slang
+
+struct Base
+{
+    int a;
+}
+
+struct Derived : Base
+{}
+
+int getA(Derived d) { return d.a; }