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diff --git a/src/go/types/fixedbugs/issue6977.src b/src/go/types/fixedbugs/issue6977.src
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+++ b/src/go/types/fixedbugs/issue6977.src
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+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "io"
+
+// Alan's initial report.
+
+type I interface { f(); String() string }
+type J interface { g(); String() string }
+
+type IJ1 = interface { I; J }
+type IJ2 = interface { f(); g(); String() string }
+
+var _ = (*IJ1)(nil) == (*IJ2)(nil) // static assert that IJ1 and IJ2 are identical types
+
+// The canonical example.
+
+type ReadWriteCloser interface { io.ReadCloser; io.WriteCloser }
+
+// Some more cases.
+
+type M interface { m() }
+type M32 interface { m() int32 }
+type M64 interface { m() int64 }
+
+type U1 interface { m() }
+type U2 interface { m(); M }
+type U3 interface { M; m() }
+type U4 interface { M; M; M }
+type U5 interface { U1; U2; U3; U4 }
+
+type U6 interface { m(); m /* ERROR duplicate method */ () }
+type U7 interface { M32 /* ERROR duplicate method */ ; m() }
+type U8 interface { m(); M32 /* ERROR duplicate method */ }
+type U9 interface { M32; M64 /* ERROR duplicate method */ }
+
+// Verify that repeated embedding of the same interface(s)
+// eliminates duplicate methods early (rather than at the
+// end) to prevent exponential memory and time use.
+// Without early elimination, computing T29 may take dozens
+// of minutes.
+type (
+        T0 interface { m() }
+        T1 interface { T0; T0 }
+        T2 interface { T1; T1 }
+        T3 interface { T2; T2 }
+        T4 interface { T3; T3 }
+        T5 interface { T4; T4 }
+        T6 interface { T5; T5 }
+        T7 interface { T6; T6 }
+        T8 interface { T7; T7 }
+        T9 interface { T8; T8 }
+
+        T10 interface { T9; T9 }
+        T11 interface { T10; T10 }
+        T12 interface { T11; T11 }
+        T13 interface { T12; T12 }
+        T14 interface { T13; T13 }
+        T15 interface { T14; T14 }
+        T16 interface { T15; T15 }
+        T17 interface { T16; T16 }
+        T18 interface { T17; T17 }
+        T19 interface { T18; T18 }
+
+        T20 interface { T19; T19 }
+        T21 interface { T20; T20 }
+        T22 interface { T21; T21 }
+        T23 interface { T22; T22 }
+        T24 interface { T23; T23 }
+        T25 interface { T24; T24 }
+        T26 interface { T25; T25 }
+        T27 interface { T26; T26 }
+        T28 interface { T27; T27 }
+        T29 interface { T28; T28 }
+)
+
+// Verify that m is present.
+var x T29
+var _ = x.m