Fix language server auto-complete regression in debug build. (#8416)

Fixes this regression:

```slang
struct MyType
{
    // Regression Condition 1: there must be more than one member in the lookup scope.
    float v;
    int getSum() { return 0; }
}

void m(MyType t)
{
    // Regression condition 2: the completion must be in an init expression.
    // Regression condition 3: none of the candidate members can coerce to the expected type.
    // Regression behavior: no completion candidates are shown, because
    // SemanticsVisitor::resolveOverloadedLookup throws an error when there are 0 applicable candidates
    // after type coercion filtering.
    Texture2D x = t.;  // completion request after . here
}
```

The root cause is that we shouldn't be applying candidate filtering on
the candidate list when in completion checking mode.

Closes #8417.

2 files changed, 33 insertions, 3 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index ec249f56d..8994eb783 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1139,13 +1139,20 @@ LookupResult SemanticsVisitor::resolveOverloadedLookup(
     if (!inResult.isOverloaded())
         return inResult;
 
+    // If this is a lookup for a completion request token (to generate completion candidate list),
+    // don't apply expected-type filtering on the lookup result so we can report the entire
+    // candidate list in the language server.
+    bool shouldSkipCoercionFilter =
+        getLinkage()->contentAssistInfo.checkingMode == ContentAssistCheckingMode::Completion &&
+        inResult.getName() == getSession()->getCompletionRequestTokenName();
+
     // We are going to build up a list of items to return.
     List<LookupResultItem> items;
     for (auto item : inResult.items)
     {
         // First we check if the item is coercible to targetType.
         // And skip if it doesn't.
-        if (targetType)
+        if (targetType && !shouldSkipCoercionFilter)
         {
             auto declType = GetTypeForDeclRef(item.declRef, SourceLoc());
             if (!canCoerce(targetType, declType, nullptr, nullptr))
@@ -1186,9 +1193,12 @@ LookupResult SemanticsVisitor::resolveOverloadedLookup(
     // The resulting `items` list should be all those items
     // that were neither better nor worse than one another.
     //
-    // There should always be at least one such item.
+    // If no candidates are passing coercion check, return unresolved lookup result
+    // and leave downstream logic to diagnose and error.
     //
-    SLANG_ASSERT(items.getCount() != 0);
+
+    if (items.getCount() == 0)
+        return inResult;
 
     LookupResult result;
     for (auto item : items)
diff --git a/tests/language-server/completion-in-initexpr.slang b/tests/language-server/completion-in-initexpr.slang
new file mode 100644
index 000000000..8eec82991
--- /dev/null
+++ b/tests/language-server/completion-in-initexpr.slang
@@ -0,0 +1,20 @@
+//TEST:LANG_SERVER(filecheck=CHECK):
+//COMPLETE:17,21
+struct MyType
+{
+    // Regression Condition 1: there must be more than one member in the lookup scope.
+    float v;
+    int getSum() { return 0; }
+}
+
+void m(MyType t)
+{
+    // Regression condition 2: the completion must be in an init expression.
+    // Regression condition 3: none of the candidate members can coerce to the expected type.
+    // Regression behavior: no completion candidates are shown, because
+    // SemanticsVisitor::resolveOverloadedLookup throws an error when there are 0 applicable candidates
+    // after type coercion filtering.
+    Texture2D x = t.;
+}
+
+// CHECK: getSum