[dev.typeparams] cmd/compile: export/import of recursive generic types.

Deal with export/import of recursive generic types. This includes
typeparams which have bounds that reference the typeparam.

There are three main changes:

  - Change export/import of typeparams to have an implicit "declaration"
    (doDecl). We need to do a declaration of typeparams (via the
    typeparam's package and unique name), because it may be referenced
    within its bound during its own definition.

  - We delay most of the processing of the Instantiate call until we
    finish the creation of the top-most type (similar to the way we
    delay CheckSize). This is because we can't do the full instantiation
    properly until the base type is fully defined (with methods). The
    functions delayDoInst() and resumeDoInst() delay and resume the
    processing of the instantiations.

  - To do the full needed type substitutions for type instantiations
    during import, I had to separate out the type subster in stencil.go
    and move it to subr.go in the typecheck package. The subster in
    stencil.go now does node substitution and makes use of the type
    subster to do type substitutions.

Notable other changes:
 - In types/builtins.go, put the newly defined typeparam for a union type
   (related to use of real/imag, etc.) in the current package, rather
   than the builtin package, so exports/imports work properly.

 - In types2, allowed NewTypeParam() to be called with a nil bound, and
   allow setting the bound later. (Needed to import a typeparam whose
   bound refers to the typeparam itself.)

 - During import of typeparams in types2 (importer/import.go), we need
   to keep an index of the typeparams by their package and unique name
   (with id). Use a new map typParamIndex[] for that. Again, this is
   needed to deal with typeparams whose bounds refer to the typeparam
   itself.

 - Added several new tests absdiffimp.go and orderedmapsimp.go. Some of
   the orderemapsimp tests are commented out for now, because there are
   some issues with closures inside instantiations (relating to unexported
   names of closure structs).

 - Renamed some typeparams in test value.go to make them all T (to make
   typeparam uniqueness is working fine).

Change-Id: Ib47ed9471c19ee8e9fbb34e8506907dad3021e5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/323029
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

2 files changed, 104 insertions, 0 deletions

diff --git a/test/typeparam/absdiffimp.dir/a.go b/test/typeparam/absdiffimp.dir/a.go
new file mode 100644
index 0000000000..df81dcf538
--- /dev/null
+++ b/test/typeparam/absdiffimp.dir/a.go
@@ -0,0 +1,75 @@
+// Copyright 2021 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 a
+
+import (
+	"math"
+)
+
+type Numeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64 |
+		~complex64 | ~complex128
+}
+
+// numericAbs matches numeric types with an Abs method.
+type numericAbs[T any] interface {
+	Numeric
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func absDifference[T numericAbs[T]](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// orderedNumeric matches numeric types that support the < operator.
+type orderedNumeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64
+}
+
+// Complex matches the two complex types, which do not have a < operator.
+type Complex interface {
+	~complex64 | ~complex128
+}
+
+// orderedAbs is a helper type that defines an Abs method for
+// ordered numeric types.
+type orderedAbs[T orderedNumeric] T
+
+func (a orderedAbs[T]) Abs() orderedAbs[T] {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+// complexAbs is a helper type that defines an Abs method for
+// complex types.
+type complexAbs[T Complex] T
+
+func (a complexAbs[T]) Abs() complexAbs[T] {
+	r := float64(real(a))
+	i := float64(imag(a))
+	d := math.Sqrt(r * r + i * i)
+	return complexAbs[T](complex(d, 0))
+}
+
+// OrderedAbsDifference returns the absolute value of the difference
+// between a and b, where a and b are of an ordered type.
+func OrderedAbsDifference[T orderedNumeric](a, b T) T {
+	return T(absDifference(orderedAbs[T](a), orderedAbs[T](b)))
+}
+
+// ComplexAbsDifference returns the absolute value of the difference
+// between a and b, where a and b are of a complex type.
+func ComplexAbsDifference[T Complex](a, b T) T {
+	return T(absDifference(complexAbs[T](a), complexAbs[T](b)))
+}
diff --git a/test/typeparam/absdiffimp.dir/main.go b/test/typeparam/absdiffimp.dir/main.go
new file mode 100644
index 0000000000..8eefdbdf38
--- /dev/null
+++ b/test/typeparam/absdiffimp.dir/main.go
@@ -0,0 +1,29 @@
+// Copyright 2021 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 main
+
+import (
+	"a"
+	"fmt"
+)
+
+func main() {
+	if got, want := a.OrderedAbsDifference(1.0, -2.0), 3.0; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := a.OrderedAbsDifference(-1.0, 2.0), 3.0; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := a.OrderedAbsDifference(-20, 15), 35; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+
+	if got, want := a.ComplexAbsDifference(5.0+2.0i, 2.0-2.0i), 5+0i; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := a.ComplexAbsDifference(2.0-2.0i, 5.0+2.0i), 5+0i; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+}