Fix lowering of extern types with defaults. (#6512)

* Fix lowering of `extern` types with defaults.

* Fix test.

* Fix test.

Author: csyonghe

source/slang/slang-lower-to-ir.cpp

@@ -8193,7 +8193,8 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
         // Make sure that all the entries in the witness table have been filled in,
         // including any cases where there are sub-witness-tables for conformances
         bool isExplicitExtern = false;
-        if (!isImportedDecl(context, parentDecl, isExplicitExtern))
+        bool isImported = isImportedDecl(context, parentDecl, isExplicitExtern);
+        if (!isImported || isExplicitExtern)
         {
             Dictionary<WitnessTable*, IRWitnessTable*> mapASTToIRWitnessTable;
             lowerWitnessTable(

tests/bugs/gh-6504-linker.slang

@@ -0,0 +1,118 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv
+
+// CHECK: OpEntryPoint
+
+static extern const bool _AlphaTest = false;
+static extern const bool _BaseColorMap = false;
+static extern const bool _NormalMap = false;
+static extern const bool _EmissiveMap = false;
+static extern const bool _MetallicRoughnessMap = false;
+
+interface IVertexInput
+{
+    float3 GetPosition();
+    float3 GetNormal();
+    float4 GetTangent();
+    float2 GetUV();
+    float4 GetBone();
+}
+
+struct StandardVertexInput : IVertexInput
+{
+    float3 position;
+    float3 normal;
+    float4 tangent;
+    float2 uv;
+
+    float3 GetPosition() {return position;}
+    float3 GetNormal() {return normal;}
+    float4 GetTangent() {return tangent;}
+    float2 GetUV() { return uv;}
+    float4 GetBone() { return float4(0,0,0,0);}
+}
+
+extern struct VertexInput : IVertexInput = StandardVertexInput;
+
+struct PushConstant
+{
+    float4x4 model;
+    float4x4 invTspModel;
+} 
+
+[vk_push_constant]
+PushConstant pconst;
+
+struct VertexOutput
+{
+    float4 position : SV_Position;
+    float3 positionWS;
+    float3 normalWS;
+    float3 tangentWS;
+    float3 bitangentWS;
+    float2 uv;
+}
+
+
+
+VertexOutput SceneLitVertex(VertexInput input)
+{
+    VertexOutput output;
+    float3 vertexPos = input.GetPosition();
+    float3 normal = input.GetNormal();
+    float4 tangent = input.GetTangent();
+
+    output.positionWS = mul(pconst.model, float4(vertexPos, 1)).xyz;
+
+    matrix<float,3,3> model33 = (float3x3)pconst.invTspModel;
+    output.normalWS = mul(model33, normal).xyz;
+    output.tangentWS = mul(model33, tangent.xyz);
+    output.bitangentWS = tangent.w * cross(output.normalWS, output.tangentWS);
+    output.uv = input.GetUV();
+
+    float4x4 vp = {};
+    output.position = mul(vp, float4(output.positionWS, 1));
+
+    return output;
+}
+
+
+[shader("vertex")]
+VertexOutput VertexMain(VertexInput input)
+{
+    return SceneLitVertex(input);
+}
+
+struct PerMaterial
+{
+    float4 baseColorFactor;
+    float4 emissive;
+    float roughness;
+    float metallic;
+    float alphaCutoff;
+
+    Sampler2D baseColorTex;
+    Sampler2D normalMap;
+    Sampler2D metallicRoughnessMap;
+    Sampler2D emissiveMap;
+};
+
+ParameterBlock<PerMaterial> perMaterial;
+
+struct FragmentOutput
+{
+    float4 color : SV_Target0;
+}
+
+FragmentOutput SceneLitFragment(VertexOutput input)
+{
+    FragmentOutput output;
+    output.color = 0;
+
+    return output;
+}
+
+[shader("fragment")]
+FragmentOutput FragmentMain(VertexOutput input)
+{
+    return SceneLitFragment(input);
+}