[CSGen] Constraint generation for PackExpansionExpr should return a null type

if the constraint system has not recorded a type for an expanded pack element
type repr.

Author: hborla

lib/Sema/CSGen.cpp

@@ -3078,6 +3078,12 @@ namespace {
         if (auto *elementExpr = getAsExpr<PackElementExpr>(pack)) {
           packType = CS.getType(elementExpr->getPackRefExpr());
         } else if (auto *elementType = getAsTypeRepr<PackElementTypeRepr>(pack)) {
+          // OpenPackElementType sets types for 'each T' type reprs in
+          // expressions. Some invalid code won't make it there, and
+          // the constraint system won't have recorded a type.
+          if (!CS.hasType(elementType->getPackType()))
+            return Type();
+
           packType = CS.getType(elementType->getPackType());
         } else {
           llvm_unreachable("unsupported pack reference ASTNode");

test/Constraints/pack-expansion-expressions.swift

@@ -70,6 +70,7 @@ func typeReprPacks<each T>(_ t: repeat each T) where each T: ExpressibleByIntege
 
   _ = Array<each T>() // expected-error {{pack reference 'T' can only appear in pack expansion or generic requirement}}
   _ = 1 as each T // expected-error {{pack reference 'T' can only appear in pack expansion or generic requirement}}
+  repeat Invalid<String, each T>("") // expected-error {{cannot find 'Invalid' in scope}}
 }
 
 func sameShapeDiagnostics<each T, each U>(t: repeat each T, u: repeat each U) {