Avoid suggesting omitting the LHS type of a var decl for known weaker types.

This arises when type checking fails, and golint cannot determine
that the LHS and RHS are truly of different types. In that situation
it is an annoying false positive.
diff --git a/lint.go b/lint.go
index 2899b29..66fc69f 100644
--- a/lint.go
+++ b/lint.go
@@ -887,6 +887,18 @@
 	"0i":  true,
+// knownWeakerTypes is a set of types that are commonly used to weaken var declarations.
+// A common form of var declarations that legitimately mentions an explicit LHS type
+// is where the LHS type is "weaker" than the exact RHS type, where "weaker" means an
+// interface compared to a concrete type, or an interface compared to a superset interface.
+// A canonical example is `var out io.Writer = os.Stdout`.
+// This is only used when type checking fails to determine the exact types.
+var knownWeakerTypes = map[string]bool{
+	"io.Reader":     true,
+	"io.Writer":     true,
+	"proto.Message": true,
 // lintVarDecls examines variable declarations. It complains about declarations with
 // redundant LHS types that can be inferred from the RHS.
 func (f *file) lintVarDecls() {
@@ -932,7 +944,7 @@
 				return false
-			// The next two conditions are for suppressing the warning in situations
+			// The next three conditions are for suppressing the warning in situations
 			// where we were unable to typecheck.
 			// If the LHS type is an interface, don't warn, since it is probably a
@@ -946,6 +958,14 @@
 			if defType, ok := f.isUntypedConst(rhs, scope); ok && !isIdent(v.Type, defType) {
 				return false
+			// If the LHS is a known weaker type, and we couldn't type check both sides,
+			// don't warn.
+			if lhsTyp == nil || rhsTyp == nil {
+				if knownWeakerTypes[f.render(v.Type)] {
+					return false
+				}
+			}
 			f.errorf(v.Type, 0.8, category("type-inference"), "should omit type %s from declaration of var %s; it will be inferred from the right-hand side", f.render(v.Type), v.Names[0])
 			return false
diff --git a/testdata/var-decl.go b/testdata/var-decl.go
index 5921e39..d382bee 100644
--- a/testdata/var-decl.go
+++ b/testdata/var-decl.go
@@ -63,6 +63,16 @@
 // LHS is a different type than the RHS.
 var myStringer fmt.Stringer = q(0)
+// We don't figure out the true types of LHS and RHS here,
+// but io.Writer is a known weaker type for many common uses,
+// so the suggestion should be suppressed here.
+var out io.Writer = os.Stdout
+// This next one, however, should be type checked.
+var out2 io.Writer = newWriter() // MATCH /should.*io\.Writer/
+func newWriter() io.Writer { return nil }
 var y string = q(1).String() // MATCH /should.*string/
 type q int