2532 – '=' does not give a boolean result

Using a boolean assignment in an if condition e.g. as follows gives IMHO an unjustified error message. When a new variable is declared in the if() condition, everything is OK:
// tst.d
void main()
{ bool b;
 if(b=false) { b = true; }
 if(bool c=false) { c = true; }
}
$ dmd tst
tst.d(4): Error: '=' does not give a boolean result

= does not give a boolean result for good reason: it's to avoid the common mistake in C/C++ where:
if(x = 5) { ... }
is used when:
if(x == 5) { ... }
was meant.

It works as designed. It is designed to avoid mistakes.
"if (var = expr) {" is disallowed on purpose to avoid mistakes where you inteded to write "if (var == expr) {":
int a = ...;
if (a = 42) { // oops, what a typo! I ment if (a == 42) here...
Bool is no different from int in this expression.
However, "if (type var == expression) {" is not a valid construct, so there is no chance to make a mistake and thus "if (type var = expression) {" is allowed.
There is no 'special cases' for bools.

The text of the message is not true if the variable being assigned to is ALREADY a bool. I know that the syntax form 'if (a = b)' is to avoided, but to say that, IN THIS CASE, the result is not a boolean is plainly wrong.
bool a;
bool b;
b = (a = true); // A boolean expression (a = true) is used to assign to 'b'.
As we know that (a = b) is a boolean expression, it seems wrong to say that it is a boolean EXCEPT in an 'if' construct. We do this to no other data type.
So maybe the wording of the message can be changed to reflect what is actually being disallowed. Something like ...
"The syntax form 'if (a = b)' is not allowed".
That takes the argument away from the datatype issue and makes it a syntax issue instead.

Might "expression is not a valid condition" be better?

Reopening as an enhancement.

Added keyword diagnostic.

Hmm... After all D has switched to strict booleans?

*** Issue 4163 has been marked as a duplicate of this issue. ***

https://github.com/D-Programming-Language/dmd/pull/122
A better message for all?
Error: assignment cannot be used as a condition

https://github.com/D-Programming-Language/dmd/commit/6279cb917ba926d2b2a14c28b23ff8e00871be27
https://github.com/D-Programming-Language/dmd/commit/772ff9a6009731cd509385d1d365d91398d39f7c
https://github.com/D-Programming-Language/dmd/commit/2f69b157fe06218cab0ebe6cf93d2f474d50a32f