cmd/compile/internal/types2: fix type inference

Don't let type parameters that are not filled in with concrete
type arguments escape from constraint type inference - such
inferred types are not "real".

While at it, implement a tparamsList.String method for debugging.

Fixes #45548.

Change-Id: I40f13ff7af08d0357a5c66234bfcdd0b7ed5fdd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/311651
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/fixedbugs/issue45548.go2

@@ -0,0 +1,13 @@
+// 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 p
+
+func f[F interface{type *Q}, G interface{type *R}, Q, R any](q Q, r R) {}
+
+func _() {
+	f[*float64, *int](1, 2)
+	f[*float64](1, 2)
+	f(1, 2)
+}

src/cmd/compile/internal/types2/infer.go

@@ -451,6 +451,25 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type, report bool) (ty
 		dirty = dirty[:n]
 	}
 
+	// Once nothing changes anymore, we may still have type parameters left;
+	// e.g., a structural constraint *P may match a type parameter Q but we
+	// don't have any type arguments to fill in for *P or Q (issue #45548).
+	// Don't let such inferences escape, instead nil them out.
+	for i, typ := range types {
+		if typ != nil && isParameterized(tparams, typ) {
+			types[i] = nil
+		}
+	}
+
+	// update index
+	index = -1
+	for i, typ := range types {
+		if typ == nil {
+			index = i
+			break
+		}
+	}
+
 	return
 }
 

src/cmd/compile/internal/types2/unify.go

@@ -6,6 +6,8 @@
 
 package types2
 
+import "bytes"
+
 // The unifier maintains two separate sets of type parameters x and y
 // which are used to resolve type parameters in the x and y arguments
 // provided to the unify call. For unidirectional unification, only
@@ -69,6 +71,22 @@ type tparamsList struct {
 	indices []int // len(d.indices) == len(d.tparams)
 }
 
+// String returns a string representation for a tparamsList. For debugging.
+func (d *tparamsList) String() string {
+	var buf bytes.Buffer
+	buf.WriteByte('[')
+	for i, tname := range d.tparams {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		writeType(&buf, tname.typ, nil, nil)
+		buf.WriteString(": ")
+		writeType(&buf, d.at(i), nil, nil)
+	}
+	buf.WriteByte(']')
+	return buf.String()
+}
+
 // init initializes d with the given type parameters.
 // The type parameters must be in the order in which they appear in their declaration
 // (this ensures that the tparams indices match the respective type parameter index).