Short-circuit validation of #if conditions after a versioned check.

When we encounter a check like `#if compiler(>=6.0) && something` or
`#if swift(<6.0) || something`, and the left-hand term is a versioning
check that determines the result of the whole condition, then we will
not attempt to validate the right-hand term. This allows us to use
versioned checks along with new discovery features (say, if we add an
`#if attribute(x)`) without having to next conditions.

Author: DougGregor

lib/Parse/ParseIfConfig.cpp

@@ -133,6 +133,13 @@ static Expr *getSingleSubExp(ArgumentList *args, StringRef kindName,
   return nullptr;
 }
 
+/// Returns \c true if the condition is a version check.
+static bool isVersionIfConfigCondition(Expr *Condition);
+
+/// Evaluate the condition.
+/// \c true if success, \c false if failed.
+static bool evaluateIfConfigCondition(Expr *Condition, ASTContext &Context);
+
 /// The condition validator.
 class ValidateIfConfigCondition :
   public ExprVisitor<ValidateIfConfigCondition, Expr*> {
@@ -202,7 +209,7 @@ class ValidateIfConfigCondition :
 
     // We will definitely be consuming at least one operator.
     // Pull out the prospective RHS and slice off the first two elements.
-    Expr *RHS = validate(S[1]);
+    Expr *RHS = S[1];
     S = S.slice(2);
 
     while (true) {
@@ -226,7 +233,7 @@ class ValidateIfConfigCondition :
 
       OpName = NextOpName;
       Op = S[0];
-      RHS = validate(S[1]);
+      RHS = S[1];
       S = S.slice(2);
     }
 
@@ -426,14 +433,37 @@ class ValidateIfConfigCondition :
     return E;
   }
 
+  Expr *visitBinaryExpr(BinaryExpr *E) {
+    auto OpName = getDeclRefStr(E->getFn(), DeclRefKind::BinaryOperator);
+    if (auto lhs = validate(E->getLHS())) {
+      // If the left-hand side is a versioned condition, skip evaluation of
+      // the right-hand side if it won't ever affect the result.
+      if (OpName && isVersionIfConfigCondition(lhs)) {
+        assert(*OpName == "&&" || *OpName == "||");
+        bool isLHSTrue = evaluateIfConfigCondition(lhs, Ctx);
+        if (isLHSTrue && *OpName == "||")
+          return lhs;
+        if (!isLHSTrue && *OpName == "&&")
+          return lhs;
+      }
+
+      E->getArgs()->setExpr(0, lhs);
+    }
+
+    if (auto rhs = validate(E->getRHS()))
+      E->getArgs()->setExpr(1, rhs);
+
+    return E;
+  }
+
   // Fold sequence expression for non-Swift3 mode.
   Expr *visitSequenceExpr(SequenceExpr *E) {
     ArrayRef<Expr*> Elts = E->getElements();
-    Expr *foldedExpr = validate(Elts[0]);
+    Expr *foldedExpr = Elts[0];
     Elts = Elts.slice(1);
     foldedExpr = foldSequence(foldedExpr, Elts);
     assert(Elts.empty());
-    return foldedExpr;
+    return validate(foldedExpr);
   }
 
   // Other expression types are unsupported.
@@ -608,7 +638,6 @@ class IsVersionIfConfigCondition :
   bool visitExpr(Expr *E) { return false; }
 };
 
-/// Returns \c true if the condition is a version check.
 static bool isVersionIfConfigCondition(Expr *Condition) {
   return IsVersionIfConfigCondition().visit(Condition);
 }

test/Parse/unknown_platform.swift

@@ -0,0 +1,28 @@
+// RUN: %target-typecheck-verify-swift
+
+// expected-error@+1{{unexpected platform condition}}
+#if hasGreeble(blah)
+#endif
+
+// Future compiler, short-circuit right-hand side
+#if compiler(>=10.0) && hasGreeble(blah)
+#endif
+
+// Current compiler, short-circuit right-hand side
+#if compiler(<10.0) || hasGreeble(blah)
+#endif
+
+// This compiler, don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if compiler(>=5.7) && hasGreeble(blah)
+#endif
+
+// This compiler, don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if compiler(<5.8) || hasGreeble(blah)
+#endif
+
+// Not a "version" check, so don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if os(macOS) && hasGreeble(blah)
+#endif