Add static assertion that uint32_t is unsigned int or wider #818

1. 
   real-or-random commented at 11:36 am on September 17, 2020: contributor

   Solves one item in #792 .
2. Add static assertion that uint32_t is unsigned int or wider c0041b5cfc
3. in src/assumptions.h:28 in c0041b5cfc

   20@@ -19,7 +21,11 @@ struct secp256k1_assumption_checker {
   21        allowed. */
   22     int dummy_array[(
   23         /* Bytes are 8 bits. */
   24-        CHAR_BIT == 8 &&
   25+        (CHAR_BIT == 8) &&
   26+
   27+        /* No integer promotion for uint32_t. This ensures that we can multiply uintXX_t values where XX >= 32
   28+           without signed overflow, which would be undefined behaviour. */
   29+        (UINT_MAX <= UINT32_MAX) &&
   

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   roconnor-blockstream commented at 2:50 pm on September 17, 2020:

   I think INT_MAX < UINT32_MAX would more directly capture your intention here; however given the other rules of C, I do admit that what you have written ends up being equivalent.

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   real-or-random commented at 11:42 am on September 18, 2020:

   Yeah, I think our intuition is just different. My thought was that if uint32_t is at least as large as unsigned int, then no promotion can happen at all, neither to signed int nor to unsigned int.

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   roconnor-blockstream commented at 12:06 pm on September 18, 2020:

   I think of the rule as if every value of a type can fit within a signed int, then it is subject to integer promotion.

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   sipa commented at 8:52 pm on September 18, 2020:

   Both seem reasonable and harmless to me.

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   real-or-random commented at 9:56 am on September 19, 2020:

   Wanna ACK then?
4. elichai approved
5. 
   elichai commented at 7:40 am on September 21, 2020: contributor

   Looks good. ACK c0041b5cfca5efb160aa9a5616350069c89a8c29
6. 
   sipa commented at 3:41 am on September 26, 2020: contributor

   utACK c0041b5cfca5efb160aa9a5616350069c89a8c29
7. real-or-random merged this on Sep 26, 2020
8. real-or-random closed this on Sep 26, 2020

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9. jasonbcox referenced this in commit 7ccaa00435 on Sep 29, 2020
10. deadalnix referenced this in commit 7516ca138d on Sep 30, 2020

Contributors
real-or-random roconnor-blockstream sipa elichai