[Parse] Disallow unsupported condition expression in #if directive

In non-Swift3 mode.

Previously:

  #if FOO = false
  #elseif FOO ? false : true
  #endif

were silently accepted.
i.e. '= false' and '? false : true' were silently ignored.

Author: rintaro

lib/Parse/ParseStmt.cpp

@@ -1570,10 +1570,22 @@ Parser::classifyConditionalCompilationExpr(Expr *condition,
               break;
           }
         } else {
-          D.diagnose(SE->getLoc(),
-                     diag::unsupported_conditional_compilation_binary_expression);
+          D.diagnose(
+              SE->getLoc(),
+              diag::unsupported_conditional_compilation_binary_expression);
           return ConditionalCompilationExprState::error();
         }
+      } else {
+        // Swift3 didn't have this branch. the operator and the RHS are
+        // silently ignored.
+        if (!Context.isSwiftVersion3()) {
+          D.diagnose(
+              elements[iOperator]->getLoc(),
+              diag::unsupported_conditional_compilation_expression_type);
+          return ConditionalCompilationExprState::error();
+        } else {
+          // TODO: Emit a warning.
+        }
       }
 
       iOperator += 2;

test/Compatibility/conditional_compiliation_expr.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift -D FOO -swift-version 3
+
+
+#if FOO = false
+undefinedFunc() // expected-error {{use of unresolved identifier 'undefinedFunc'}}
+#else
+undefinedFunc() // ignored.
+#endif
+
+#if false
+
+#elseif !FOO ? false : true
+undefinedFunc() // ignored.
+#else
+undefinedFunc() // expected-error {{use of unresolved identifier 'undefinedFunc'}}
+#endif

test/Parse/ConditionalCompilation/basicParseErrors.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -D FOO -D BAZ
+// RUN: %target-typecheck-verify-swift -D FOO -D BAZ -swift-version 4
 
 #if FOO == BAZ // expected-error{{expected '&&' or '||' expression}}
 var x = 0
@@ -88,3 +88,18 @@ fn_j() // OK
 
 #if foo || bar || nonExistent() // expected-error {{expected only one argument to platform condition}}
 #endif
+
+#if FOO = false
+// expected-error @-1 {{invalid conditional compilation expression}}
+undefinedFunc() // ignored.
+#else
+undefinedFunc() // expected-error {{use of unresolved identifier 'undefinedFunc'}}
+#endif
+
+#if false
+#elseif FOO ? true : false
+// expected-error @-1 {{invalid conditional compilation expression}}
+undefinedFunc() // ignored.
+#else
+undefinedFunc() // expected-error {{use of unresolved identifier 'undefinedFunc'}}
+#endif