[ConstraintSystem] Use lightweight conformance check in determining w…

lib/Sema/TypeCheckConstraints.cpp

@@ -1972,15 +1972,21 @@ Expr *PreCheckExpression::simplifyTypeConstructionWithLiteralArg(Expr *E) {
   Type type;
   if (typeExpr->getTypeLoc().wasValidated()) {
     type = typeExpr->getTypeLoc().getType();
-  } else if (auto *rep = typeExpr->getTypeRepr()) {
+  } else {
     TypeResolutionOptions options(TypeResolverContext::InExpression);
     options |= TypeResolutionFlags::AllowUnboundGenerics;
-    auto resolution = TypeResolution::forContextual(DC);
-    type = resolution.resolveType(rep, options);
-    typeExpr->getTypeLoc().setType(type);
+
+    auto &typeLoc = typeExpr->getTypeLoc();
+    bool hadError =
+        TC.validateType(typeLoc, TypeResolution::forContextual(DC), options);
+
+    if (hadError)
+      return nullptr;
+
+    type = typeLoc.getType();
   }
 
-  if (!type)
+  if (!type || !type->getAnyNominal())
     return nullptr;
 
   // Don't bother to convert deprecated selector syntax.
@@ -1989,16 +1995,13 @@ Expr *PreCheckExpression::simplifyTypeConstructionWithLiteralArg(Expr *E) {
       return nullptr;
   }
 
-  ConformanceCheckOptions options;
-  options |= ConformanceCheckFlags::InExpression;
-  options |= ConformanceCheckFlags::SkipConditionalRequirements;
-
-  auto result = TypeChecker::conformsToProtocol(type, protocol, DC, options);
-  if (!result || !result->isConcrete())
-    return nullptr;
-
-  return CoerceExpr::forLiteralInit(TC.Context, argExpr, call->getSourceRange(),
-                                    typeExpr->getTypeLoc());
+  auto *NTD = type->getAnyNominal();
+  SmallVector<ProtocolConformance *, 2> conformances;
+  return NTD->lookupConformance(DC->getParentModule(), protocol, conformances)
+             ? CoerceExpr::forLiteralInit(TC.Context, argExpr,
+                                          call->getSourceRange(),
+                                          typeExpr->getTypeLoc())
+             : nullptr;
 }
 
 /// Pre-check the expression, validating any types that occur in the

test/Constraints/generics.swift

@@ -757,3 +757,16 @@ func test_generic_subscript_with_missing_arg() {
     _ = s[0] // expected-error {{generic parameter 'U' could not be inferred}}
   }
 }
+
+func rdar_50007727() {
+  struct A<T> { // expected-note {{'T' declared as parameter to type 'A'}}
+    struct B<U> : ExpressibleByStringLiteral {
+      init(stringLiteral value: String) {}
+    }
+  }
+
+  struct S {}
+  let _ = A.B<S>("hello")
+  // expected-error@-1 {{generic parameter 'T' could not be inferred in cast to 'A.B'}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{12-12=<Any>}}
+}