[Sema] Assert we don't type-check bindings independently of closures

Bindings in closures must be type-checked together with the surrounding
closure, add an assertion to make sure we don't try this. Carve out
an exception for code completion which may still kick lazy
type-checking on failure.

Author: hamishknight

lib/Sema/TypeCheckConstraints.cpp

@@ -841,6 +841,18 @@ bool TypeChecker::typeCheckBinding(Pattern *&pattern, Expr *&initializer,
                 initializer, DC, patternType, pattern,
                 /*bindPatternVarsOneWay=*/false);
 
+  if (auto *CE = dyn_cast<ClosureExpr>(DC)) {
+    if (!PBD || !isPlaceholderVar(PBD)) {
+      // Completion may trigger lazy type-checking as part of fallback
+      // unqualified lookup, just decline to type-check.
+      if (CE->getASTContext().SourceMgr.hasIDEInspectionTargetBuffer()) {
+        target.markInvalid();
+        return true;
+      }
+      ABORT("Cannot type-check PatternBindingDecl without closure context");
+    }
+  }
+
   // Type-check the initializer.
   auto resultTarget = typeCheckExpression(target, options);
 

test/IDE/complete_unqualified_fallback.swift

@@ -47,8 +47,14 @@ S {
     S {
       S {
         S {
+          let x = 0
           #^COMPLETE^#
-          // COMPLETE: Decl[Struct]/CurrModule: FooBar[#FooBar#]; name=FooBar
+          // COMPLETE-DAG: Decl[Struct]/CurrModule: FooBar[#FooBar#]; name=FooBar
+
+          // We decline to type-check a variable indepedently of the surrounding
+          // closure, so this has an ErrorType. If this changes, you need to
+          // make this test case more complex.
+          // COMPLETE-DAG: Decl[LocalVar]/Local: x[#_#]; name=x
         }
       }
     }