cmd/compile: treat constants to type parameter conversion as non-constant in Unified IR

Fixes #54307

Change-Id: Idcbdb3b1cf7c7fd147cc079659f29a9b5d17e6e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/421874
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/noder/helpers.go

@@ -251,3 +251,8 @@ func idealType(tv types2.TypeAndValue) types2.Type {
 	}
 	return typ
 }
+
+func isTypeParam(t types2.Type) bool {
+	_, ok := t.(*types2.TypeParam)
+	return ok
+}

src/cmd/compile/internal/noder/reader.go

@@ -2019,6 +2019,7 @@ func (r *reader) expr() (res ir.Node) {
 		typ := r.typ()
 		pos := r.pos()
 		typeWord, srcRType := r.convRTTI(pos)
+		dstTypeParam := r.Bool()
 		x := r.expr()
 
 		// TODO(mdempsky): Stop constructing expressions of untyped type.
@@ -2034,12 +2035,21 @@ func (r *reader) expr() (res ir.Node) {
 			base.ErrorExit() // harsh, but prevents constructing invalid IR
 		}
 
-		n := ir.NewConvExpr(pos, ir.OCONV, typ, x)
+		ce := ir.NewConvExpr(pos, ir.OCONV, typ, x)
-		n.TypeWord, n.SrcRType = typeWord, srcRType
+		ce.TypeWord, ce.SrcRType = typeWord, srcRType
 		if implicit {
-			n.SetImplicit(true)
+			ce.SetImplicit(true)
 		}
-		return typecheck.Expr(n)
+		n := typecheck.Expr(ce)
+
+		// spec: "If the type is a type parameter, the constant is converted
+		// into a non-constant value of the type parameter."
+		if dstTypeParam && ir.IsConstNode(n) {
+			// Wrap in an OCONVNOP node to ensure result is non-constant.
+			n = Implicit(ir.NewConvExpr(pos, ir.OCONVNOP, n.Type(), n))
+			n.SetTypecheck(1)
+		}
+		return n
 	}
 }
 

src/cmd/compile/internal/noder/writer.go

@@ -1692,6 +1692,7 @@ func (w *writer) expr(expr syntax.Expr) {
 			w.typ(tv.Type)
 			w.pos(expr)
 			w.convRTTI(w.p.typeOf(expr.ArgList[0]), tv.Type)
+			w.Bool(isTypeParam(tv.Type))
 			w.expr(expr.ArgList[0])
 			break
 		}
@@ -1854,6 +1855,7 @@ func (w *writer) implicitConvExpr(pos poser, dst types2.Type, expr syntax.Expr)
 		w.typ(dst)
 		w.pos(pos)
 		w.convRTTI(src, dst)
+		w.Bool(isTypeParam(dst))
 		// fallthrough
 	}
 	w.expr(expr)

test/fixedbugs/issue54307.go

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2022 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[Int int, Uint uint]() {
+	_ = uint(Int(-1))
+	_ = uint(Uint(0) - 1)
+}
+
+func g[String string]() {
+	_ = String("")[100]
+}
+
+var _ = f[int, uint]
+var _ = g[string]