[CSGen] Detect and diagnose invalid pack expansion expressions

Detect that pack expansion expression doesn't have any pack
references during constraint generation.

Resolves: rdar://107835215

Author: xedin

lib/Sema/CSFix.cpp

@@ -2759,7 +2759,8 @@ DestructureTupleToMatchPackExpansionParameter::create(
 
 bool AllowValueExpansionWithoutPackReferences::diagnose(
     const Solution &solution, bool asNote) const {
-  return false;
+  ValuePackExpansionWithoutPackReferences failure(solution, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AllowValueExpansionWithoutPackReferences *

lib/Sema/CSGen.cpp

@@ -3081,6 +3081,12 @@ namespace {
       // pack expansion expression through the shape type variable.
       SmallVector<ASTNode, 2> expandedPacks;
       expr->getExpandedPacks(expandedPacks);
+
+      if (expandedPacks.empty()) {
+        (void)CS.recordFix(AllowValueExpansionWithoutPackReferences::create(
+            CS, CS.getConstraintLocator(expr)));
+      }
+
       for (auto pack : expandedPacks) {
         Type packType;
         if (auto *elementExpr = getAsExpr<PackElementExpr>(pack)) {

test/Constraints/pack-expansion-expressions.swift

@@ -447,3 +447,26 @@ do {
     // expected-error@-1 {{pack reference 'each T' can only appear in pack expansion}}
   }
 }
+
+// rdar://107835215 - failed to produce a diagnostic for invalid pack expansion expression
+do {
+  func test1(x: Int) {
+    repeat x
+    // expected-error@-1:5 {{value pack expansion must contain at least one pack reference}}
+  }
+
+  func test2<T: Numeric>(_ x: T) {
+    repeat print(x * 2)
+    // expected-error@-1:5 {{value pack expansion must contain at least one pack reference}}
+  }
+
+  struct S<T> {
+    init(_: T) {}
+  }
+
+  func test<each T>(x: repeat each T, y: Int) {
+    func f<each A, each B>(_: repeat each A, y: repeat each B) {}
+    f(repeat each x, y: repeat [S(y)])
+    // expected-error@-1:25 {{value pack expansion must contain at least one pack reference}}
+  }
+}