[CSBindings] Don't delay inferred loeading-dot base inference if source is a contextual type

Transitively inferring a protocol type binding from a contextual
type means that the binding set for a leading-dot member reference
is complete because there could be no other sources of bindings
when expression is connected directly to a contextual type.

Resolves: rdar://145103149

Author: xedin

lib/Sema/CSBindings.cpp

@@ -129,9 +129,15 @@ bool BindingSet::isDelayed() const {
       if (Bindings.empty())
         return true;
 
-
-      if (Bindings[0].BindingType->is<ProtocolType>())
-        return true;
+      if (Bindings[0].BindingType->is<ProtocolType>()) {
+        auto *locator = Bindings[0].getLocator();
+        // If the binding got inferred from a contextual type
+        // this set shouldn't be delayed because there won't
+        // be any other inference sources for this leading-dot
+        // syntax member.
+        if (!locator->findLast<LocatorPathElt::ContextualType>())
+          return true;
+      }
     }
 
     // Since force unwrap preserves l-valueness, resulting

test/Constraints/result_builder.swift

@@ -1402,3 +1402,33 @@ func testBuildFinalResultDependentOnContextualType() {
 
 testBuildFinalResultDependentOnContextualType()
 // CHECK: Optional(42)
+
+protocol TestLeadingDot {
+}
+
+@resultBuilder
+struct IntBuilder {
+  static func buildBlock(_ v: Int) -> Int {
+    print("buildBlock: \(v)")
+    return v
+  }
+}
+
+extension TestLeadingDot where Self == NoopImpl {
+  static func test(@IntBuilder builder: () -> Int) -> NoopImpl {
+    builder()
+    return NoopImpl()
+  }
+}
+
+struct NoopImpl : TestLeadingDot {
+}
+
+func testLeadingDotSyntax(v: Int) {
+  let x: some TestLeadingDot = .test {
+    v
+  }
+}
+
+testLeadingDotSyntax(v: -42)
+// CHECK: buildBlock: -42