Infer the return types of local functions where appropriate.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index 34a3e3dc720021be87b60adb1dc94f049c926e6a..f9adf852bee599f65b52c93efca7ccb788279804 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -22,6 +22,7 @@ import 'package:kernel/ast.dart'
         DynamicType,
         Expression,
         Field,
+        FunctionNode,
         FunctionType,
         Initializer,
         InterfaceType,
@@ -33,10 +34,12 @@ import 'package:kernel/ast.dart'
         ProcedureKind,
         PropertyGet,
         PropertySet,
+        ReturnStatement,
         Statement,
         SuperMethodInvocation,
         SuperPropertyGet,
         SuperPropertySet,
+        TypeParameter,
         TypeParameterType,
         VariableDeclaration,
         VoidType;
@@ -637,6 +640,136 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     return inferredType;
   }
 
+  DartType inferLocalFunction(FunctionNode function, DartType typeContext,

[Siggi Cherem (dart-lang), 2017/06/21 22:34:27]

I assume this just moved, but otherwise is the same?

[Paul Berry, 2017/06/22 17:44:20]

There are two small changes.  See comments below.

+      bool typeNeeded, int fileOffset, DartType returnContext, bool isNamed) {

[Paul Berry, 2017/06/22 17:44:20]

Previously, returnContext was always initially `null`.  Now it is passed in as a
parameter.  In the old code path (KernelFunctionExpression._inferExpression), we
pass in `null`, so the behavior is unchanged.  In the new code path
(KernelFunctionDeclaration._inferStatement), we pass in the explicitly declared
return type, or `null` if there isn't one.  This allows the new code path to
supply the declared return type without having to synthesize 
an ersatz typeContext to contain it.

+    bool hasImplicitReturnType = returnContext == null;
+    if (!isTopLevel) {
+      for (var parameter in function.positionalParameters) {
+        if (parameter.initializer != null) {
+          inferExpression(parameter.initializer, parameter.type, false);
+        }
+      }
+      for (var parameter in function.namedParameters) {
+        if (parameter.initializer != null) {
+          inferExpression(parameter.initializer, parameter.type, false);
+        }
+      }
+    }
+
+    // Let `<T0, ..., Tn>` be the set of type parameters of the closure (with
+    // `n`=0 if there are no type parameters).
+    List<TypeParameter> typeParameters = function.typeParameters;
+
+    // Let `(P0 x0, ..., Pm xm)` be the set of formal parameters of the closure
+    // (including required, positional optional, and named optional parameters).
+    // If any type `Pi` is missing, denote it as `_`.
+    List<VariableDeclaration> formals = function.positionalParameters.toList()
+      ..addAll(function.namedParameters);
+
+    // Let `B` denote the closure body.  If `B` is an expression function body
+    // (`=> e`), treat it as equivalent to a block function body containing a
+    // single `return` statement (`{ return e; }`).
+
+    // Attempt to match `K` as a function type compatible with the closure (that
+    // is, one having n type parameters and a compatible set of formal
+    // parameters).  If there is a successful match, let `<S0, ..., Sn>` be the
+    // set of matched type parameters and `(Q0, ..., Qm)` be the set of matched
+    // formal parameter types, and let `N` be the return type.
+    Substitution substitution;
+    List<DartType> formalTypesFromContext =
+        new List<DartType>.filled(formals.length, null);
+    if (strongMode && typeContext is FunctionType) {
+      for (int i = 0; i < formals.length; i++) {
+        if (i < function.positionalParameters.length) {
+          formalTypesFromContext[i] =
+              getPositionalParameterType(typeContext, i);
+        } else {
+          formalTypesFromContext[i] =
+              getNamedParameterType(typeContext, formals[i].name);
+        }
+      }
+      returnContext = typeContext.returnType;
+
+      // Let `[T/S]` denote the type substitution where each `Si` is replaced with

[Siggi Cherem (dart-lang), 2017/06/21 22:34:27]

nit: wrap

[Paul Berry, 2017/06/22 17:44:20]

Done.

+      // the corresponding `Ti`.
+      var substitutionMap = <TypeParameter, DartType>{};
+      for (int i = 0; i < typeContext.typeParameters.length; i++) {
+        substitutionMap[typeContext.typeParameters[i]] =
+            i < typeParameters.length
+                ? new TypeParameterType(typeParameters[i])
+                : const DynamicType();
+      }
+      substitution = Substitution.fromMap(substitutionMap);
+    } else {
+      // If the match is not successful because  `K` is `_`, let all `Si`, all
+      // `Qi`, and `N` all be `_`.
+
+      // If the match is not successful for any other reason, this will result in
+      // a type error, so the implementation is free to choose the best error
+      // recovery path.
+      substitution = Substitution.empty;
+    }
+
+    // Define `Ri` as follows: if `Pi` is not `_`, let `Ri` be `Pi`.
+    // Otherwise, if `Qi` is not `_`, let `Ri` be the greatest closure of
+    // `Qi[T/S]` with respect to `?`.  Otherwise, let `Ri` be `dynamic`.
+    for (int i = 0; i < formals.length; i++) {
+      KernelVariableDeclaration formal = formals[i];
+      if (KernelVariableDeclaration.isImplicitlyTyped(formal)) {
+        DartType inferredType;
+        if (formalTypesFromContext[i] != null) {
+          inferredType = greatestClosure(coreTypes,
+              substitution.substituteType(formalTypesFromContext[i]));
+        } else {
+          inferredType = const DynamicType();
+        }
+        instrumentation?.record(Uri.parse(uri), formal.fileOffset, 'type',
+            new InstrumentationValueForType(inferredType));
+        formal.type = inferredType;
+      }
+    }
+
+    // Let `N'` be `N[T/S]`.  The [ClosureContext] constructor will adjust
+    // accordingly if the closure is declared with `async`, `async*`, or
+    // `sync*`.
+    if (returnContext != null) {
+      returnContext = substitution.substituteType(returnContext);
+    }
+
+    // Apply type inference to `B` in return context `N’`, with any references
+    // to `xi` in `B` having type `Pi`.  This produces `B’`.
+    bool isExpressionFunction = function.body is ReturnStatement;
+    bool needToSetReturnType = hasImplicitReturnType &&

[Paul Berry, 2017/06/22 17:44:20]

Previously this was:

bool needToSetReturnType = isExpressionFunction || strongMode;

In the old code path, hasImplicitReturnType is `true` and isNamed is `false`, so
the behavior is unchanged.  In the new code path, isNamed is `true` and
hasImplicitReturnType is determined by the presence or absence of an explicit
return type.  This ensures that (a) we don't try to change the return type if
one was explicitly given, and (b) in spec mode, we don't try to infer the return
type of named functions at all.

+        ((isExpressionFunction && !isNamed) || strongMode);
+    ClosureContext oldClosureContext = this.closureContext;
+    ClosureContext closureContext =
+        new ClosureContext(this, function.asyncMarker, returnContext);
+    this.closureContext = closureContext;
+    inferStatement(function.body);
+
+    // If the closure is declared with `async*` or `sync*`, let `M` be the least
+    // upper bound of the types of the `yield` expressions in `B’`, or `void` if
+    // `B’` contains no `yield` expressions.  Otherwise, let `M` be the least
+    // upper bound of the types of the `return` expressions in `B’`, or `void`
+    // if `B’` contains no `return` expressions.
+    DartType inferredReturnType;
+    if (needToSetReturnType || typeNeeded) {
+      inferredReturnType =
+          closureContext.inferReturnType(this, isExpressionFunction);
+    }
+
+    // Then the result of inference is `<T0, ..., Tn>(R0 x0, ..., Rn xn) B` with
+    // type `<T0, ..., Tn>(R0, ..., Rn) -> M’` (with some of the `Ri` and `xi`
+    // denoted as optional or named parameters, if appropriate).
+    if (needToSetReturnType) {
+      instrumentation?.record(Uri.parse(uri), fileOffset, 'returnType',
+          new InstrumentationValueForType(inferredReturnType));
+      function.returnType = inferredReturnType;
+    }
+    this.closureContext = oldClosureContext;
+    return typeNeeded ? function.functionType : null;
+  }
+
   /// Performs the core type inference algorithm for method invocations (this
   /// handles both null-aware and non-null-aware method invocations).
   DartType inferMethodInvocation(