add special case for min/max, fixes #281

[email protected]

Review URL: https://codereview.chromium.org/1253903004 .

Author: John Messerly

lib/src/checker/resolver.dart

@@ -637,6 +637,29 @@ class RestrictedStaticTypeAnalyzer extends StaticTypeAnalyzer {
         }
       }
     }
+
+    // Pretend dart:math's min and max are generic:
+    //
+    //     T min<T extends num>(T x, T y);
+    //
+    // and infer T. In practice, this just means if the type of x and y are
+    // both double or both int, we treat that as the return type.
+    //
+    // The Dart spec has similar treatment for binary operations on numbers.
+    //
+    // TODO(jmesserly): remove this when we have a fix for
+    // https://github.com/dart-lang/dev_compiler/issues/28
+    if (isDartMathMinMax(e)) {
+      var args = node.argumentList.arguments;
+      if (args.length == 2) {
+        var tx = args[0].staticType;
+        var ty = args[1].staticType;
+        if (tx == ty &&
+            (tx == _typeProvider.intType || tx == _typeProvider.doubleType)) {
+          node.staticType = tx;
+        }
+      }
+    }
   }
 
   void _inferObjectAccess(

lib/src/dart_sdk.dart

@@ -171,5 +171,7 @@ final Map<String, String> mockSdkSources = {
         class Random {
           bool nextBool() {}
         }
+        num min(num x, num y) {}
+        num max(num x, num y) {}
         ''',
 };

lib/src/utils.dart

@@ -452,3 +452,7 @@ String errorCodeName(ErrorCode errorCode) {
 bool isInlineJS(Element e) => e is FunctionElement &&
     e.library.source.uri.toString() == 'dart:_foreign_helper' &&
     e.name == 'JS';
+
+bool isDartMathMinMax(Element e) => e is FunctionElement &&
+    e.library.source.uri.toString() == 'dart:math' &&
+    (e.name == 'min' || e.name == 'max');

test/checker/checker_test.dart

@@ -2764,5 +2764,42 @@ void main() {
         Iterable<int> baz5() sync* { yield* (/*info:InferredTypeAllocation*/new Iterable()); }
     '''
             }));
+
+    test(
+        'dart:math min/max',
+        () => testChecker({
+              '/main.dart': '''
+        import 'dart:math';
+
+        void printInt(int x) => print(x);
+        void printDouble(double x) => print(x);
+
+        num myMax(num x, num y) => max(x, y);
+
+        main() {
+          // Okay if static types match.
+          printInt(max(1, 2));
+          printInt(min(1, 2));
+          printDouble(max(1.0, 2.0));
+          printDouble(min(1.0, 2.0));
+
+          // No help for user-defined functions from num->num->num.
+          printInt(/*warning:DownCastImplicit*/myMax(1, 2));
+          printInt(myMax(1, 2) as int);
+
+          // Mixing int and double means return type is num.
+          printInt(/*warning:DownCastImplicit*/max(1, 2.0));
+          printInt(/*warning:DownCastImplicit*/min(1, 2.0));
+          printDouble(/*warning:DownCastImplicit*/max(1, 2.0));
+          printDouble(/*warning:DownCastImplicit*/min(1, 2.0));
+
+          // Types other than int and double are not accepted.
+          printInt(
+              /*warning:DownCastImplicit*/min(
+                  /*severe:StaticTypeError*/"hi",
+                  /*severe:StaticTypeError*/"there"));
+        }
+    '''
+            }));
   });
 }