4 files changed, 53 insertions, 1 deletions
diff --git a/src/cmd/compile/internal/base/debug.go b/src/cmd/compile/internal/base/debug.go
index f1d020f342..5edb665e37 100644
--- a/src/cmd/compile/internal/base/debug.go
+++ b/src/cmd/compile/internal/base/debug.go
@@ -34,6 +34,7 @@ type DebugFlags struct {
 	NoRefName            int    `help:"do not include referenced symbol names in object file"`
 	PCTab                string `help:"print named pc-value table\nOne of: pctospadj, pctofile, pctoline, pctoinline, pctopcdata"`
 	Panic                int    `help:"show all compiler panics"`
+	Reshape              int    `help:"print information about expression reshaping"`
 	Slice                int    `help:"print information about slice compilation"`
 	SoftFloat            int    `help:"force compiler to emit soft-float code"`
 	SyncFrames           int    `help:"how many writer stack frames to include at sync points in unified export data"`
diff --git a/src/cmd/compile/internal/noder/codes.go b/src/cmd/compile/internal/noder/codes.go
index fb4fb4a886..c1ee8d15c5 100644
--- a/src/cmd/compile/internal/noder/codes.go
+++ b/src/cmd/compile/internal/noder/codes.go
@@ -58,6 +58,7 @@ const (
 	exprNil
 	exprFuncInst
 	exprRecv
+	exprReshape
 )
 
 type codeAssign int
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 5f770166db..cf1e1440df 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -89,7 +89,7 @@ func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pk
 	}
 }
 
-// A writer provides APIs for reading an individual element.
+// A reader provides APIs for reading an individual element.
 type reader struct {
 	pkgbits.Decoder
 
@@ -2367,6 +2367,41 @@ func (r *reader) expr() (res ir.Node) {
 		typ := r.exprType()
 		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, typ))
 
+	case exprReshape:
+		typ := r.typ()
+		x := r.expr()
+
+		if types.IdenticalStrict(x.Type(), typ) {
+			return x
+		}
+
+		// Comparison expressions are constructed as "untyped bool" still.
+		//
+		// TODO(mdempsky): It should be safe to reshape them here too, but
+		// maybe it's better to construct them with the proper type
+		// instead.
+		if x.Type() == types.UntypedBool && typ.IsBoolean() {
+			return x
+		}
+
+		base.AssertfAt(x.Type().HasShape() || typ.HasShape(), x.Pos(), "%L and %v are not shape types", x, typ)
+		base.AssertfAt(types.Identical(x.Type(), typ), x.Pos(), "%L is not shape-identical to %v", x, typ)
+
+		// We use ir.HasUniquePos here as a check that x only appears once
+		// in the AST, so it's okay for us to call SetType without
+		// breaking any other uses of it.
+		//
+		// Notably, any ONAMEs should already have the exactly right shape
+		// type and been caught by types.IdenticalStrict above.
+		base.AssertfAt(ir.HasUniquePos(x), x.Pos(), "cannot call SetType(%v) on %L", typ, x)
+
+		if base.Debug.Reshape != 0 {
+			base.WarnfAt(x.Pos(), "reshaping %L to %v", x, typ)
+		}
+
+		x.SetType(typ)
+		return x
+
 	case exprConvert:
 		implicit := r.Bool()
 		typ := r.typ()
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index c2c3567220..75ff000249 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -1629,6 +1629,16 @@ func (w *writer) expr(expr syntax.Expr) {
 			w.typ(tv.Type)
 			return
 		}
+
+		// With shape types (and particular pointer shaping), we may have
+		// an expression of type "go.shape.*uint8", but need to reshape it
+		// to another shape-identical type to allow use in field
+		// selection, indexing, etc.
+		if typ := tv.Type; !tv.IsBuiltin() && !isTuple(typ) && !isUntyped(typ) {
+			w.Code(exprReshape)
+			w.typ(typ)
+			// fallthrough
+		}
 	}
 
 	if obj != nil {
@@ -2199,6 +2209,11 @@ func isUntyped(typ types2.Type) bool {
 	return ok && basic.Info()&types2.IsUntyped != 0
 }
 
+func isTuple(typ types2.Type) bool {
+	_, ok := typ.(*types2.Tuple)
+	return ok
+}
+
 func (w *writer) itab(typ, iface types2.Type) {
 	typ = types2.Default(typ)
 	iface = types2.Default(iface)